1) After you declare the attack, once it is legal, the attacker exhausts and the attack begins.
2) Yep
3) Barry Allen <> The Flash reads:
Whenever Barry Allen becomes powered-up while attacking a character, if
Barry Allen is unequipped, you may ready him. If you do, he can't cause
breakthrough this turn.
When Barry get's powered up, he readies - this is before you do stunning and adding up damage. Now, should The Flash be stunned prior to the resolution of his effect, he would not ready unless you could recover him first, as stunned characters cannot ready.
4) Yep