Last I saw him was in the Batman “Hush” arc, and I don’t recall seeing him die there, but it’s been a while since I read it.
He died in Death and the Maidens #9, which appeared after the end of “Hush”.
Ra’s returning from the dead would be about as surprising as Jean Grey returning from the dead. His entire schtick is that he has a string of rejuvenating pits. Yeah, supposedly they’re all sealed. So Batman missed one. Still, I’m not sure why Ra’s would hide himself from his paid and blackmailed underlings like that.
Now, who is Mockingbird?
I’m guessing Luthor. He’s twice now protected the Six from direct lethal assaults by the core council. Instead he sends B-Listers, and brings them in alive. He could have planted the bug in the plane himself. Why? I’m not sure about that part yet. Perhaps to make sure that somehow, he’ll have all the players under his thumb. Perhaps to act as a counter against the society himself, should he lose control of it.
Second guess is Catman. He’s our POV character, so it’s the biggest shock, we know he’s hiding something (why he’s suddenly cool), and we only have his word that his lions were killed. He may not have a direct and simple motivation, as he claims.