Lots of ways to handle the second part, but for the first part:
AoO trigger immediately, they don’t just “untrigger” b/c of a defend action. I would rule all the other AoO happen as well. Now of course, if they are all surrounding him, not all of them would actually get an AoO in the first place, b/c they would move next ot a few of them on their way out, and then they could do the defend and be free from attacks on those they are now next to.
You would explain the optional choices the same way you always can. It’s all happening in the chaos of battle, so as the NPC is running away,the player manages to get the immobile bane off, but it is delayed, so the NPC gets roughly 10’ away.
Note: Allowing a player who fails on a Bane attack to then choose the very bane they were attempting is… not right. You shouldn’t do that (see below), and truthfully, the “choose 3” option was meant just for damaging attacks, but it seems the wording isn’t consistent for that in the rules presently. I would have allowed knockdown instead of immobile I suppose.
Then again, not sure I would did the success with a twist for AoO, I would just do failure but the story progresses (the NPC gets away).
Reason a player shouldn’t get to pick the very bane they failed is, well, you can just always do that, and then always succeed with immobile no matter what. Sure, the target might get to do something back to you, but you still got 100% what you wanted on them. Changing to another bane makes more sense I feel, and can narratively make sense as well (Attempting to lock up their body, you miss your mark and instead cause them to trip on their footing. The result causes you strain and you take [3 damage or a bane that makes sense])
Narratively, I could see a few times where it could make sense, so I might allow it once in awhile, but if the player was attempting to abuse that, probably would stop it.