As for Hathaway not being in one episode there is previous for this as Lewis didn't appear in one episode of Morse, the one where Morse was in hospital and tried to solve a Victorian murder.
If KW wants to come back but LF doesn't then they could just do a Midsommer Murders and give Lewis a new sidekick
Kevin actually decided to leave Morse at that time, which was why he didn't do the Wench is Dead. He wasn't going to do anymore.
He only went back cos the one after that was the last one.
I cant see Kevin doing it without Lawrence, even as one offs, not when he said about 3 years ago that he'd only go on doing Lewis as long as Lawrence wanted to do it.