I don't think it's sunken cost, more like he has no other option at all. He is doing the best he can with what he has. Even with his memories he doesn't have a place to return because the system has juridiction over espers and guides and Wui Wu told the truth about mutants being put to death. The best he can hope for with his family is to put them all in danger going against the military and making a scandal in the media. That's the best case scenario and it's not something he would likely chose because it's not great. Even if Wui Wu had set him free, MC would have been caught and put to death in his next medical exam... So... Yeah, find a way to get good with the pysicho or die. The only other options are to accept being the psycho's doll or be made a doll against his will, like Wui Wu treatened many times
The psychological horror aspect of the reveal that his memories had a gaping hole in them completely blindsided me despite all the hints leading up to it. In a way, it is a cop out to diffuse responsibility and shift blame to a version of ML that can't be held accountable, but the plot twist was an interesting one even if the conflict gets glossed over quickly in service of story progression.
What stands out to me is the conversation it opens about personal agency, dehumanization, and molding a partner to fit expectations. Would he still be with him if his memories were in tact? Is a choice you were manipulated into making really your choice? Are they still together only because of sunken cost fallacy?
The writing takes full advantage of branching timelines and flips the story on its head by pulling the rug out from under MC who thinks he possesses more power than he actually does. I found the betrayal so deeply and deliberately haunting that I keep turning over the scenario. Consent that is given without being privy to all the facts of reality can hardly be considered consent.