Why did Elijah and Moses appear beside Jesus if the Bible says no one can return?
When they say “no one can return”, they're talking about reincarnation, which was a belief some of the other religions have. The God of the Bible needs us Christians to be clear on this: no matter how much the reincarnation buffs try to present evidence to the contrary, God is NOT re-sending people to be born on earth a second time.
That said, of course God can send a vision of a soul from Heaven to Earth any time he wants to, since this isn't the same thing at all. I suspect Moses and Elijah weren't in normal human bodies, but were just part of a vision that “disappeared” to the human observers as soon as the vision was through. So no “meat suits” were involved at any time in this case.