Regarding crosses/vampires etc.

This is how I see it.

Their high concept allows Conviction based maneuvers, blocks or even attacks if you've got an implement of your faith (pentacle, cross, star of david, whatever), defended with monster discipline most likely, although perhaps sometimes some other attribute.  You an also just skip the maneuver/block/attack and burn fate to invoke their high concept for immediate effect but that gives them the fate point to hose you with later, so if you're a high conviction fellow like Dresden or Father Fordhill, you'd most likely try the maneuver or block first.

Interpreted this way, the whole "you have to have faith for a cross to work" schtick makes perfect sense - low conviction folks by definition lack the kind of power to block a RCV with a cross the same way low discipline folks are unlikely to avoid being addicted by their venom, and low strength people unable to break their grasp.    But if you burn fate and invoke the high concept, that's saying that in this one moment in time,  you have enough faith to hold them off, even though most of the time a cross is just jewelry to you.   (perhaps for the first time in your life you sincerely pray to a higher power for protection, or whatever, but it doesn't stick with you unless you buy aspects and/or conviction later at milestones)

I agree that some objects seem to "store" faith (holy water being the most obvious example), letting anybody screw with certain high concepts if they have an ally with faith.  If you have a real "True Faith" person in the group, they might, for example, be able to bless the sword of the swashbuckler type, and have it beat holy item based catches, although mechanically it's likely handled by the faith person doing a conviction maneuver with a free tag passed to the swashbuckler, meaning you'll need to burn fate after the first free tag (or tags, depending on how the GM sets up the difficulty for a conviction maneuver of this sort...might work until the scene is over to beat catch, but you can only invoke for +2 or reroll once without fate...)

If he has control of +8 but his rote was based on a total power of +7 (for one stress).  He could take that rote and cast it at Power 8 and take a 2 stress hit and still control the spell. 

Indeed. I pretty much always go this route with rotes.  You've only got 4 stress boxes, and it's almost wasteful not to spend 2 stress on a spell (as that also neatly becomes a minor mental consequence if you don't have any stress boxes left).  If I've got 4 rotes, I tend to have one be 1 stress, two at 2 stress, one at 4 stress just to give me the most options for playing with stress boxes and consequences.  After the first casting, the 2 stress ones are just as cheap as the 1 stress rote, and having one big reliable zap for either the 4th stress box, 2 minor consequences (if I have 5 conviction) or a moderate consequence seems pretty worthwhile....if I need a conviction+3 spell I'd want an option for one that doesn't risk fizzling with fallout or hosing me with physical stress on backlash.

I'll often use a dedicated offensive control foci on the 4 stress power, and just be routinely able to control anything else at conviction+1.  All of this assumes I'm running a disciplined mage of course.  If I'm running a Dresden-style brute I might not have all that many rotes to begin with and might be more willing to accept fallout and such, only really getting the full use of my power when invoking aspects on the control side.

Yeah, the GM can do it (when I collapsed the entire Hollywood sign hill in my Live and Die in LA Game and caused a real earthquake with the citywide "Waiting for the Big One" aspect in response to a player using an "earth tremble" type spell that's what I was doing.  I guess I owed one PC caught in it a fate point, but everyone else affected was NPCs.  Everything else it did to the encounters going on was cosmetic and interesting, but not at the level of a compel.)

Had Ian (the player) attempted the same thing he'd have had to pay similar costs for a zone-wide maneuver (which lets you do a free tag on the entire zone), and he'd have to pay for the size of the entire city, which would be something only likely to be achieved with major ritual magic.  (as Cowl did when he hexed the entire city of Chicago).

One advantage of being the GM is you get as many fate points as you need to have something cool happen.  Players have a fair amount of narrative power, but not to that degree.   A player could invoke "Waiting for the Big One" to have a minor earthquake happen city-wide with a single fate point, but it would only have a significant effect on one visible character (where in my scene it endangered a dozen or so police officers and one PC).

If it's cool enough, of course, the GM can always step in with whatever fate is needed.  To some extent, so can sponsored magic, but that kind of debt is often career-ending, similar to a death curse.

The rule of thumb for a bad experience in 3.5 or Pathfinder Organized Play (eg, Living Greyhawk, Pathfinder Society) was you had to have two of three things true.

1.  Bad gm
2.  Bad module
3.  Bad mix of players at the table

A fun group of players and a module that is well written can overcome a poorly prepared GM stumbling through the encounters and box-text descriptions because they'll chew on the scenery, make their own fun and nobody will suddenly die by a GM mistake in the combat encounters or have the adventure derail because the PCs are competent and the module's organized assuming at least some people will be running it cold.

A good gm and a good module can generally adapt it in minor ways on the fly to the foibles of dysfunctional, incompetent or contentious PCs and give a pretty good experience anyway, drawing them into the game.

A good GM and good players will have a pretty good game, no matter what the author of the module did, and I've seen some doozies there.  My favorite anecdote along these lines had a fairly normal adventure start an encounter with the gm just going "sorry about this" and having the following monsters attack our river boat....

Flying awakened (intelligent) squid barbarians, juiced up on potions wielding multiple axes in their tentacles.  The table was so busy busting up in laughter that they almost killed us all, because we just couldn't take it seriously.   My wife's reaction when I told her later was "Squids are SALT-WATER critters!"  (we'd seen enough goofy stuff that THIS was what offended her).

Mostly you get mediocre in two of the three, but one of them good, and that is also pretty fun.  (and if the players are mediocre, well, you're part of the problem.  Start injecting some fun!).  As long as it isn't actively bad, most gaming is a bit like a good meal or sex.  Even when it isn't great, it's pretty good.

Now in most games the GM is the author of the story, so a bad GM can kill a game pretty quick.  Fate doesn't have the crutch of pre-planned modules, because it doesn't lend itself to that (it has scenes that after the initial hook are pretty much determined by player actions in the first scene - at best the GM has some milestones and stuff he kind of wants the PC's to interact with, but in the end they drive things much more than D&D/Pathfinder).   This means the GM also has to be an author, and so do the players.  If either side of this equation breaks down, Fate can be pretty stale.   (see my other posts on the difficulties of running Fate in a convention setting, based on my anecdotal early experiences.  It can be done, but it's harder if the GM is unprepared or inexperienced with the fate economy, or if the pregens are poorly thought out)

I was an experienced GM but not with fate when I ran a Codex Alera adaption for one guy good at Fate, one with passing experience and two newbies.  We had fun because they were fans of the setting and I'm a decent GM, but I didn't give them enough aspects that fit the scenes well on the character sheet or in the scenes themselves, and we didn't use the fate economy very well (this adaption was like DFRPG though in that the non-fate add-ons were strong enough the flavor got through, even if the aspects were weak, which is probably why it worked)

I'm an experienced player, but had an inexperienced GM and a tired, cranky table using stock Spirit of the Century characters when I had a bad experience playing.  I didn't understand my character, ran dry on fate points and got taken out in the firs encounter, got no help from anyone on the table using the aspect mechanics in terms of compels from GM and other players, was playing a social pilot in a scenario mostly underground and with nobody to talk to, so had little help from the skills part of my character sheet either.

I salvaged that game experience for myself on a break by just getting my head right and loosening up and looking for ways to chew on scenery to have some fun (which in Fate, tends to work well).  In the climactic scene I used my "fear of flying" consequences from the first encounter to hose a bunch of flying enemy psychic things that had latched into my brain, and the "spirit of freedom" high concept to judo the BBEG who thought that me letting him into my head to do the fear thing would get me to become a thrall.

Running DFRPG here is really helping a lot to have this all sink in, although I'm not totally clear on the pacing of the fate economy.  My players are mostly accumulating fate points while coping with various challenges, and we'll see how well I did when they reach the climax of this milestone - I've got that worked out, but I've got a lot less feel for how it'd go than in a normal game, given how compels and invocations can suddenly shift the situation.   One really nice thing though is that the city building effort gave me a ton of stuff to work with when responding to their actions and looking for conflicts and challenges.  The world building I usually have to do to GM my own scenario is mostly covered by the Dresden setting + city/character building, the rest fills in during play.

My interpretation is that powers do add to the mediocre. I'd think something with inhuman (and beyond) reflexes/speed would have a one-up on pure mortal speed, even when completely caught off guard.

The way to simulate that without invoking an aspect is with a stunt.  They can't normally engage their speed if they don't know anything is happening after all.   The Dresden books have plenty of examples where this is the case.  Remember that somebody really fast will likely have supporting aspects and can always spend a fate point to invoke "Son of Hermes" to react anyway during an ambush, invoking the aspect.  If you lack such an aspect, well, sure you're a Red Court Infected but your inhuman speed isn't at the same level as your fellow Red Court Infected who took the aspect "Faster than you Think".   You let yourself be ambushed or try to find a different aspect to invoke that will help.

If you want it reliable, you want a stunt -

Stunt: "Plenty of Time to React" - use Athletics instead of Alertness for the "Avoiding Surprise" trapping.  (super-speed adds to athletics for all purposes, so it'd improve this stunt, and the one best at avoiding surprise would be an individual with lots of refresh spent in super speed AND high athletics)

Likewise simulating surprise caused by a speedster zipping in from behind full cover and doing something awful before your neurons could fire can also be handled via a stunt

Stunt: "No time to react" - use Athletics instead of Stealth for the "Ambush" trapping (the enemy still has to start out unaware of you, but that's a lot easier when you start the exchange two zones away, still and behind full cover, etc.   Possibly with a fate point you could invoke this (or some other supporting speedster aspect) even when they are aware of you...they know you're there but are as helpless in the first exchange as if they didn't because you move so fast.)

Harry says a lot of things that are based on his training and he's not always right.  Magic circles are more efficient FOR HIM if they're round.  It isn't clear a different practitioner would see any difference between a circle or an oval.

What is clear is that the practitioner's belief is important ("Blue is for safety" says Harry in one book, and for him blue is good for that kind of thing because he sees it that way.  Another person may prefer green, or black or white).   It's also clear that the thing you're attempting to affect with the circle matters, what it cares about (eg, using iron to trap a faerie).    Gems, expensive stuff in time or money seem to bolster the raw power of the circle but don't really change what it does.   Symbols can (the Loup Garou circle seemed to have a physical ward, a purely magical circle of the typical sort and an extra layer for critters made of both, like a loup Garou.  I sometimes wonder if the physical and magical wards were to keep OUTSIDERS from releasing the beast by accident, as opposed to anything intended to stop the critter.

Ok, I'm going to go out on a limb here and just say it.

Don't start your Fate/DFRPG experience with a wizard.  Yes, it's powerful but if you don't have system mastery your vast range of options can grind things to a halt, plus it can be oddly fragile if you don't know how to manage your resources of both mental stress to power your spells and physical stress to soak damage+properly managing consequences.

Also you'll be blindsided by how aspect compels work, most likely.   Think of it as playing a 3.5 D&D Druid summoner with dire tiger animal companion, starting it at level 9 and handing it to a first-time player, along with a pile of source-books giving him a few score spells to decide to put in his slots, and a couple hundred critters he can summon, but with non-standard statblocks because he also has augment summoning.  It isn't likely to be the best intro to D&D for the player, or for the party.

Plus for all their power, a wizard really isn't much better than anybody else at getting things "done" in-game.  They just have a wider variety of flashy tools in any given situation.  The mortal with maxed diplomacy and resource type skills will often be better - just think about how Marcone seems to always have the upper hand over Dresden and it's generally Dresden going to Marcone for help, not the other way around - especially after the first couple of books when Marcone gets clued into the supernatural world and has had time to bring his resources to bear on the problem.  Still, sometimes even Marcone gets compelled by his prior choices and needs rescue.  Nobody wins them all in Fate.

The character you described would probably work better as just somebody with supernatural powers and skills.  That plus the aspects (which actually are a very large part of your character's "power" - you nave narrative power in this game system, which is as important or more important than objective physical power) are more than enough to learn the first time out.

For a frankenstein type character, you can actually take the -1 refresh "dead" power, giving you an animated corpse instead of a body, just to begin with.  You would have refresh to be tough, fast, strong and being life-challenged without actually being a vampire or something would cause you plenty of problems, which will likely translate into fate points that you can use to power aspects.

Obviously a hydromancer could do a Dispel type spell and have an easy excuse for it, just as a spirit caster can easily justify an invisibility+avoidance type veil of the sort Molly uses.

You don't get it for free as a side-effect of other magic though. It's something you choose to do, just like hexing or whatever.    You can use water as a ram or water to make someone choke or water to dissolve things or water to dispel by grounding out, just as you can use fire to attack, or to set somebody on fire, or to vaporize a barrier or to "cleanse" some types of magic by burning.

It's just flavor for an effect.  Most elements can do most things, water does dispels by "running water" over the effect and "grounding it out".