Can you still drop this card if the attack that targeted you, and subsequently resolved, ended up killing Dengar?

Can he play this card to interrupt before he's taken off the board?

I'm not as familiar with IA rules as I am with x-wing and armada, but looking at the card I don't think so. It says the attack targeting you has to have resolved, and I would expect killing and removing the figure would be part of the resolution of the attack.

I'm no master, but I would say that a lot of cards have "before you are defeated" written into them. Because this one does not, I would guess (GUESS) that's not the case here.

The attack has resolved and the figure has taken damaged and is defeated, the figure is no longer on the board when this effect would take place

Going with Hatting here. If you resolve the attack and Dengar is defeated during resolution, you can't play this card anymore.

Going with Hatting here. If you resolve the attack and Dengar is defeated during resolution, you can't play this card anymore.

Yes.