1) When Tyrael became the leader of the Angiris Council, was Imperius there in a humbled state or not? Did he agree to that, or did he run, or what?
We don't know for certain if Tyrael is leader of the council. We know he took the seat of Wisdom. That doesn't mean he's automatically the leader, just because it used to be the old leader's seat. Furthermore, Tyrael explicitly states why he didn't take up the seat of Justice again... and the seat of Wisdom just happened be to vacant.
As for Imperius... only the Blizzard writers know for certain. From what we've seen and read, he's very much full of pride and anger, so there's a good chance he isn't humbled at all. He doesn't have to turn to evil (though it's cliche enough for Blizzard to jump on), but if remains on the side of good, he could cause just as much problems being the same jerk he's proven to already be.
2) No seriously, how could they still be undecided about what to do with a prominent character that they went to great pains to establish? Is he going to be in the expansions or not?
D3 is chock full of plot holes. Not exactly or anywhere near the literary level of Nostromo
, The Tale of Genji
, or Shakuntala
He was still alive when he was "de-powered" and you fought Diablo (you could clearly see him breathing). I don't know why people think he was dying. Sure, the wound was major, but Tyrael himself said it could heal... he was just concerned that Imperius was going to fight Prime Evil Diablo with that wound... but at no point did it sound like Tyrael was worried Imperius would die from that wound - just if he continued to try fighting with it.
So I'm gonna go a limb, with him still being alive before the big fight, that he pulled through. Heck... his absence from the final cinema could mean he didn't approve of Tyrael taking the seat of Wisdom (i.e. not willing to honor the decision by not being there), than because he was dead.