It seems to me Mark served as Leo's "psychic anchor;" he was sure of his identity so long as Mark was there to reinforce it, making him a sort of "authority" figure in my reading of it. In this read of things the Muppet Leo would be going along with the investigation of the muppet-asylum with its earnest Adult companion at its side.

I suggest Ruffalo instead of Kingsley precisely because Ruffalo isn't the ringleader, he's on Leo's "side" (at least up until he "isn't".) Leo is suspicious of Kingsley's involvement pretty early on, so if we want to have the best surprise "betrayal" by the adult figure I think Mark would be a better person to keep in this regard.

Also isn't it implied/stated Mark Ruffalo (as a doctor) is Leo's primary caretaker / a more important doctor? I don't quite remember to be honest.