In answer to your first question, +1 Crafting: Frequency adds one use per session per item. Increasing Crafting: Frequency does not change the math for using extra enchanted item slots for frequency; those are still worth two uses per session each.
If your goal is optimization, a good rule of thumb is Crafting: Strength is better than Crafting: Frequency until you reach your Power cap at Crafting: Strength = Lore. That's not to say Crafting: Frequency is always bad; though its utility does vary by circumstance.
There are four ways to get more than one use per session out of a single enchanted item. Reduce the Power by one to increase the Frequency by one. Have at least one point in Crafting: Frequency. Devote a second (or more) enchanted item slot to Frequency, gaining two uses per session per slot used in this way. Pay a point of mental stress to get an additional use from an item that's used all of its charges for the session. <-- Often overlooked, don't forget this option.
If you're only looking at a single enchanted item on your character, putting Refinements into Crafting: Frequency through specialization or focus item is inefficient--one Refinement = two Crafting Frequency = two more uses per session on that item. You could instead have dumped half the Refinement into two enchanted item slots, and then spent one of those to get the same effect--two more uses per session on that item, at one fourth the cost.
However, if your character does a fair amount of Crafting, and regularly carries around, say, 4-6 enchanted items, Crafting: Frequency becomes a much better bargain, since the extra uses per session count for EACH of those items. If you have a couple of points in Crafting: Frequency, you can stop using extra enchanted item slots for frequency, and instead use them for more actual items.
Shorter version of the above: Crafting: Strength is either as good or better than Crafting: Frequency in ALL cases unless you are capped. Crafting: Frequency SUCKS if you have one enchanted item; don't get it. It ROCKS if you have around 4 or more enchanted items; get a couple points of Frequency in that case.
Edit: The proper name for Crafting: Power is Crafting: Strength. /facepalm