@Chttrbx_ Chapter 85: "The council doesn't have the power to control my authority, but they have the duty of officially announcing me as "the emperor's substitute."...Felix said that there shouldn't be any problems since I'm the only princess."
So no, she is not yet the absolute authority, because it hasn't been formally announced. It's a shitty formality that should've remained a formality, but now another candidate has appeared. Not only is Anastacius another candidate, he could also reveal Jeanette as being Claude's daughter (or his own, depending on how he wants to play this, but Jeanette has always been raised to believe she is Claude's blood) and put a further wrench in the council's decision. All it takes is the anti-emperor faction convincing enough of the people at the table to vote against her and she will have to step down as acting as the emperor's substitute, bc now someone of the "purer" bloodline is present, OR they reveal Jeanette, and apparently Athy doesn't want to get Jeanette involved.
In Chapter 84, Anastacius also announces he plans to cause a rebellion, too. So yeah, Anastacius is taking the route of revolt, unless Lucas can pin him down before then, except he is already leaving. No one in the room can technically hold Anastacius from leaving, because the council has not decided who is the absolute authority (IF he is to be voted as the emperor's substitute, or he declares himself as such, he can simply say no one has the authority to keep him from leaving)...which means they can't harm a potential emperor substitute, which is presumably why Felix doesn't just chop his head off.