I agree that is an appealing theory based upon their presence at the Transfiguration, as you mentioned. (Matthew 17:3, Mark 9:4, Luke 9:30-31)
However, I still must dismiss Moses' role in Revelation 11:1-12 due to the passage you noted, (Deuteronomy 34:5-6), as well as Jude 1:9.
I got a giggle out of this:
"God took Moses, showing him the land he could not cross over to and He buried him. Did anyone actually see him die?"
Did God bury Moses alive? Creepy...
Hehee... Seriously, the Bible says Moses died and that's the Truth to me. Doornail time.
That would be creepy. My thought on that is Moses wrote those 5 books but after he was taken/died someone had to finish that very last part. Once he had been "taken", as Elijah and Enoch were, and they never saw Moses again (as God "buried" him) he would essentially be "dead" to the writer.
God had to be letting us in on that fact as He made it clear who was on the Mt. of Transfiguration with Christ.
As to why Michael is contending with the devil about the "body of Moses" I don't know. Maybe it's about his transfigured body. The bible does say Moses died but then we know he didn't as we see him in a transfigured state. It's something to think about.