I had this situation come up in a game that I played;
LS declares objective and one attacker.
DS plays an event that has the one LS attacker discarded.
LS player wants to play Admiral Ackbar.
I belive Admiral Ackbar can be played, but I am unsure of when. The engagement hasn't come to the "Declare defenders" step. Can the LS player play Admiral Ackbar after defenders are declared or even after the last declared defender is focused to strike?
Ackbar can be played during any action window in the engagement, including the one immediately after defenders are declared and the one after the last defender is focused to strike. As far as I can tell, there is no rule that stops an engagement early when all participating attacking units are eliminated. In your example (where the defender eliminates all attacking units between the declare attackers and declare defenders step), the defending player will most likely not declare any defenders (unless they're planning on using some tactics icons). In that case, playing Ackbar between declare defenders and the edge battle could be a legitimate strategy… interesting find.