I'd handle all of the horse's actions with maneuvers. If I wanted to trample someone I'd Roll Survival against their Athletics, if I win they get "getting charged by a horse" or something. If someone tried to shoot it out from under me, they'd roll Guns against my Survival, if they win I'd get a "horse shot out from under me" aspect. It's not perfect but I think it's a whole lot simpler then giving the horse stats. And I think I've figured out a more specific wording (I have yet to actually figure out a name for the horse):

-1: Summon Autumn's Horse: The first or second (up to GM) turn I'd get an "Astride Autumn's Horse" aspect and I'd roll Survival instead of Athletics for sprinting.

Some other things in addition I was thinking of (but these may make this too strong for a one point power, I'd pick one more from the following:
-The aspect "Astride Autumn's Horse" comes with one free tag
-I get to modify my Athletics (Dodging) with survival due to the fact that I'm now harder to hit
-I can move 1-zone without out taking a penalty (like Casual Movement from Inhuman Speed)

I know this power will be stronger than a stunt, but not much. My goal is something close to the power of Cloak of Shadows.

And if all else fails have your new characters just happen to be in town (let them justify it themselves; following someone, running from someone, chance, etc.) and then the events of the first game they are in will compel them to stay for some reason.

I wouldn't make webbing part of Echoes of the Beast, I'd just make it it's own power, the suggestion about an offshoot of a breath weapon sounded good to me. My reasoning is this,  Echoes gives you slightly heightened senses in one specific area and adds one trapping (or add 1 to an existing trapping) and webbing sounds more like a power than a trapping too me. I'd advise something along the lines of: +1 to alertness (Avoiding Surprise) due to spiders being able to sense air movements on their hairs, and +1 to athletics (Climbing). I like the idea of Spiderweb, but I just don't see Echoes of the Beast as the proper place to put it.

Your enhanced grapple would come from tagging the aspect "All tied up" or similar from your webbing power.

The group I play with is the personification of this problem. We have lots of people who like playing, but only about 4-5 are at a season at any one time and every once in awhile someone comes up with a new character. How we handle this is by having several subplots (based on who is playing) and then tie them all together at the end in some epic battle. If all six of you are planning on playing together, then just make up some story that includes them. If Dresden has taught me anything it's that everyone needs to be flexible and occasionally have to suspend their disbelief. As I assume you're the GM and that you at least have ideas about how the original three will be incorporated into the plot, just make something up including the other two. Hope this helps.

Let me turn this around on you: Find a rule that says at char-gen that you don't have to spend all your points. Find an example that's not a mathematical error where some character built using the char-gen rules does not spend all their points. Find a rule the says you may do so explicitly.
When they say banking points, they mean something different than what you're saying. You are talking about having points that are not spent. They aren't.

Your Story page 69: "That’s it. You’re free to fill in more details as you see fit, but that’s all you need to start playing. All the other decisions that you normally make during character creation can be done during play, revealing details about the character through the emerging story." It states clearly that you can begin play with only a skeleton of a character, I see no reason that you should be required to spend all of your points. I see no reason to hold off your points, not using them is just a waste in my opinion, but if there is some story reason for your character wanting or needing to hold off on skill points, go ahead. It's not a game breaker by any means in my opinion.

On another note: "on fire" is definitely not a consequence, unless you are suggesting, that the target remains on fire for a session or two.

I wasn't suggesting the person would be on fire the whole time he has the consequence. For example say Ron the Wizard got slashed up by a Red Court and got the Consequence "Excessive bleeding", he could get bandaged up and within the hour and stop bleeding, but the consequence would remain till the next scene because of other unsaid things that went along with the bleeding: fatigue, light headed, sore, etc. So the person on fire may be put out quickly, but the burnt flesh and the like would remain. I'm not trying to nitpick or start an argument over technicalities, just restating my point to hopefully clarify. However I will concede that rewording the consequence would make it clearer.

1) If you shoot something at him he would defend with Athletics (unless a stunt says otherwise), but if you are doing some weird pyrokinesis thing then your opponent would defend with Discipline. Others may disagree but that's my take on it.

2) Well in my opinion "On fire" is more of a consequence. You could set his sleeve or pant leg on fire to create the "singed" temporary aspect on him, or again if you're doing some odd mental attack thing then your opponent could just think he's on fire. I would agree that just "On fire" doesn't make much sense for a maneuver, however this makes sense to me. I think fire is "supposed" to be more about straightforward blasting then about finesse, so it would take a craft mage to through maneuver's around with it. another possibility is to shoot a fire bolt not to harm the enemy, but to put him off balance, or flush him out of cover.

Or "Highly Conductive"
or "stiflingly hot"
or "damn it's itchy in this thing"...

Or heck, just make it "How the hell did he get that?!"