As far as I understood, armor simply reduces the stress taken by any hit the armor applies to. So you would effectively have your dodge roll + armor against any attack.
1.
The best way to do this would indeed be aspects. you could roll guns to apply a "steady aim" aspect on yourself or discipline to apply "calm as the ocean" which you can tag to make the shot. To bypass armor altogether with an aspect though, I think you would have to spend a FATE point once you made an assessment on your opponent.
2.
Like I said before, the armor reduces the attack roll by its rating, nothing more, nothing less. So a wizard with armor:5 can still be hit by a weapon:2 pistol, if the shooter rolls an attack of 4 or better, resulting in an attack of 6. Granted, without dodging he would still only end up with 1 stress, but they can accumulate rather nicely if there are enough of them.
Plus: an armor:5 spell would be at least 10 shifts, which would mean the wizard would have to have a power of 10 on that element to only get 1 stress for casting it, more if his conviction + bonuses are less (for example: conviction+specialisation and foci at 8 would result in 3 mental stress. Any caster would think twice about using a spell like that too often.). Even then, he would have to cast it beforehand, not as a defense action, and even then it would only last 1 round, if he didn't spend more power to fuel it up after that. He would also have to match his control roll (a discipline roll for 10 shifts), and a character would have to be pretty darn powerfull to be able to pull this of without ever having to resort to backlash or fallout.
There is a difference between blocks and armor. First of all, to create an armor spell, the spellpower is divided by 2, while blocks always get the amount of power that is put into them. The key difference between block and armor though is the fact that a block is destroyed once it is surpassed while armor holds as long as it is powered up. In any case, you can simply tire your wizards out if they try to have the shield on at all times.
Look at Harry and the Flamethrower in DB. Harry was well known for his shield, so Mavra has her goons shoot burning napalm at him, and while he is able to hold back the physical component of the attack, the heat still burns his hand to a useless clump. They will not be able to shield themselves from anything someone can throw at them. You could work them from a completely different angle like trapping them instead of hurting them or blinding them temporarily, actions like that could not care less about a physical block.
3.
The armor subtracts stress from the attack by its powerlevel, just like any other armor. If the creature is hit with something that satisfies the catch, the armor doesn't count.