I have heard of GF, mollies, and guppies being used as feeders for marines but they are all apparently a McD's diet for marine fish. I believe mollies and ghost shrimp can actually be acclimated to SW.
He should starve the moray and get him used to accepting thawed frozen foods such as silversides, krill, crabs, crays, etc.. (Might need to look into a feeding stick at first or hollow tube to blow in to simulate "swimming")
Real good questions, I too have wondered this and always seem to find compromised immune system, shortened lifespan, etc.., as the answer. It sounds like bouyancy would be a big problem for SW to FW, as its normal for salties to sink like rocks when given FW dips. just my 0.02$.