Sam2: You may be right that the Ravaad held that corporeality is kfira and he was speaking about believing in it by mistake.
But where does the Rambam discuss bkfira bishogeg? I have heard the idea of “nebach an apikores is oich an apikores” attributed to much more recent people. If it was stated by the Rambam, wouldn’t it be strange not to take it from him?
If you are so sure there are “rishonim” who believe this who is it and where? The Ravad does in fact make reference to great people making mistakes. Yes mistakes.