if i cause breakthrough and choose a card such as signal flare, cant my opponent just use that card in response to me choosing it? if so, do i get to tgt a different card?

Yes and no. The target is chosen at the time of breakthrough and if there is no target, you do not get to choose a new one.

There's actually a very important difference between choosing a card and targeting it.

Firstly, an effect only ever targets if it specifically uses the word target. Targets are chosen when you play an effect, which means when you place it on the chain. And if all your targets are illegal when the effect resolves, then you negate the effect.

When you are simply told to choose a card, or a character, or anything else, this is not targeting; these choices are made as part of resolving the effect, not playing it. And as you can't have priority during the resolution of an effect, neither player can do anything between the choice and the rest of the effect.

Mystique tells you to choose a card, and so you will choose one as part of resolving her effect. The opponent will reveal his hand, you will choose a card, and then he will discard it, with no points in between to interrupt the process.

If your opponent really wants to keep his Signal Flare, he needs to play it before Mystique's effect resolves.

yup, once an effect resolves there is no pause in between where the opponent or even you can play any other effect until the entire effect resolves.

so if my opponent is dumb and doesnt use it before the attack concludes...they forfeit the flare or whatever card i choose

basically once the attack is done and breakthrough happens, once they reveal their hamd, using anything is not allowed

pretty much just remember that her ability goes on the chain *cough* stack *cough* and they can plat stuff in response to that ability being activated and what not