What if a priest has no belief whatsoever in the Real Presence?No problem, whether through ignorance or heresy.
What if this priest believes all religions are the same and believes the Real Presence is a silly superstition?Ditto. Imagine the insanity that would ensue if you had to query your priest's faith before reception to make sure you attended a valid Mass. Father, do you still believe . . . And what if he lied to you? You could never have faith about any Sacrament, ever. God wants us to receive them, he doesn't put bureaucratic obstacles in the way. Read up on the Donatist heresies, and it will help explain why this is.
What if this priest uses valid matter and form, and distributes communion simply because he sees no harm done in pretending?Precisely the same situation as if an atheist baptizes to salve the fears of a family who believes. He does intend to do as the Church does, though he believes the Church to do nothing. I know there are some good quote from Aquinas directly on these points in this post. I don’t have time to dig them up right now though, so I apologize.
Dominus Vobiscum
patent +AMDG