Just had a thought so I had to join up here to share. At the end of part 18, when they are walking away from the Palmer house, Cooper and Carrie appear to, perhaps, "wake up".  When Cooper asks "what year is it?", is it me or does his voice sound different, more like "our" Cooper? I think at that point he is "our" Cooper and Carrie is Laura, hence her reaction, which frankly I find more comforting than the idea that Coop will remain the creepy version we saw through most of part 18.

I like the other theory on here that creepy Cooper is a tulpa and our Cooper is enjoying life with Janey-E and Sonny Jim, but I have a hard time believing that Coop would allow a lesser version of himself to run around with a badge and gun.