And the most likely event is that Thaos is the one responsible that this guy is at the hearing and not another dude. He wouldn't risk having an animancer that he could not control at the hearing when his plan is to that said animancer kills the duc.
That wasn't the plan and if he wanted someone to kill the Duc he'd just have an assassin there instead of unnecessarily exposing himself. He'd have also done it earlier rather than wait and risk having someone disrupt his plans. The reason why he shows up late is because he wasn't planning on it until you fuck up his plans.
It's irrelevant to his plans because, if you hadn't stopped him at the end of the game, your little exposure would had mean absolutely nothing.
Everybody hunting for him vs you and a few other guys is nothing? The reason why you have to hunt him at the end of the game is because he disrupted his exposure with the assassination and there's no one else to do it. Without it the Duc could send an army (no short of pissed off people) after him and provide you with all the resources necessary to stomp the fucker into the ground.
I can agree that the way the cutscene where Thaos kills the duc is not handled well, as it makes Thaos to exposed (i would have to watch it again, though), but him going personally to the hearing and doing the soul change thing is perfectly logical.
No it's not, it makes no sense for a puppet master to expose himself like that except if it was an impromptu plan after being exposed.
It seems we have a heavy disagreement in the understanding of one particular event of the plot.
You claim Thaos shows up in order to save his ass and that you don't expose the leaden key so animancy gets blamed. I claim that killing the duc was his plan all along and your presence there was inconsequential.
I have certain reasons to believe is this way (in all honestly, it never even crossed my mind that Thaos actions weren't premeditated, as it seemed rather obvious to me). All he does in the city is trying to discredit animancy at the eyes of the people. This cause that the dozens become very vocal about the issue, but the decision to ban animancy is entirely in the duc's hand, who has made a hearing for all the groups in power to make a stand and give their arguments. Thaos ultimate plan requires that animancy gets discredited, as it is the way that would allow people to uncover the secret behind the gods. I really don't see Thaos, after turning on the Heritage Hill machine and make one animancer look bad with his experiments, to leave the veredict to a very reasonable man and call it a day. You go out of your way to ensure that what happens in that hearing is the worst possible event for animancy.
That's why it's so important that it is an animancer that kills the duc (you can't expect to send an assassin and cause the same effect, even if you try to shoehorn false evidency that it was an animancer)