Taking someone's place for 12 years is kinda crazy at the best of times. But... this man is a president. Presidents can not simply drive away their friends and hang out in their room or something. He'd have to "not remember" far too many supporters, businesspeople, diplomats, etc. to be even remotely believable. Being a president is no easy job and there's little reason to believe he'd be qualified at it. And there's also the fact that the writers want us to believe this guy is a cold-blooded multi-murderer in the backstory... yet a panicky mess in the Rooke murder case? I mean, yeah, people change, but the combination of the inconsistent characterisation, stupid motive, and unbelievable 12-year impersonation leaves little to like about the character and anything it touches.
The part about not being worried about specific physics is fair. I try to do the same, but sometimes things go too far and it doesn't work for me. Especially for things which are sensitive to the reveal. I got very little satisfaction in puzzling out anything that happened in the final case.
I felt Courtney's character was mostly done by the end of case 4. We learn she has a son in the fifth case, I suppose, but... that feels like an oddly deletable plot point. Sebastian's character arc certainly comes to a close in case 5, on the other hand, but he isn't that important to it. It's not the end of the world, just unsatisfying with how other final cases often tie up their core cast so nicely. I didn't really feel that about this one.