Regarding Clifford as host, my guess is it just seemed to make sense: Rizzo was Steve's main character, Gonzo Dave's, Floyd Jerry's, Bill was new, Frank was part time, and Brian had no signature character. So no one had a main character who was really "host" material. And since Kevin was around and he performed Clifford, why not give it to him?
Even though I'm contradicting myself from an earlier post, Clifford as host seemed to really do very little on the show, at least compared to all that Kermit did hosting TMS. At least he had little bearing on the show as a whole. It probably was a "Let's try a newer generation of charcaters and see how they go off" but of course they didn't so much (except Pepe) which ultimately led to the show's lackluster success. People wanted the old charcaters.
Just a theory. What do you think?