I had so much fun with this card, in combo with Leia and the Wookiee Navigator. I'd make the Wookiee part of an engagement, make sure he survives, then use Fall Back! to return Leia to my hand. Since she's leaving play, she gets captured, but also clears all focus tokens in play. Then, thanks to the Wookiee's ability, everybody that just engaged that objective can do so again.

I am still new to this game and have a question regarding this card. My cousin and I were playing and after I destroyed one of his participating units he played this card and returned it to his hand. My question is whether or not that was an accurate way to use this card?

Card effects only interact with, and can only target, cards that are in play, unless the effect text specifically refers to an out-of-play card or area. A card that “leaves play” moves from a player’s play area to the “out of play” destination indicated by card or rules text. “Remove” and “Discard” are common terms that indicate a card must leave play.

Ok so if I understand you correctly then when the unit was destroyed it was considered no longer in play. Therefore he would be unable to play this card as an effect to keep his unit card from moving to his discard pile. Correct?

Yes correct

Is their any opportunity to use this card during an engagement where you can play this card and save a unit from receiving damage/destroyed? For example in the core rule book on the last page where it outlines the rules of engagement, timing and action windows which reads as follows. Reveal edge stacks, resolve fate cards, and resolve edge battle. Player actions The player who one the edge battle resolves one strike, if able. So say your the defending player with one participating unit with a health of (2) and just lost the edge battle. The active player has a unit with a strength of (2) unit damage and would therefore destroy your unit since he has the first strike. Knowing this will be the outcome since the edge battle was lost. Could you play the event card during the action window that is after the edge resolution but before the winner of the edge battle resolves his strike?

You are correct. If you have an action framework window, you may use this card.