Question on Jyn ability as we had yesterday,

Player A had initiative and declared an attack on Jyn. Player B activated her ability and the attach destroyed GaarKhan. Does this count as activation of player A turn and player B gets to activate then?

Yes it does, Garkhan still activated this allowed Jyn to use her Interrupt which allows her to stop the action for a moment Player B could then activate Jyn as per normal or any other ready unit.

Tow, I think you answered the wrong question. Combined with a slightly confusing question since it appears to be a mirror match.

Yes, player A has used their activation and its now player B's turn. Though like Tow said, Jyn is still available.

Assuming that Jyn killed the whole deployment card that activated player A is done, and it is now player B's turn. Jyn has not activated (unless you already activated her that turn). If it was against a squad of storm troopers or something the rest of the troopers would still go.