Coalize, it is good that you question this. I am intrigued by the story as well. As an aside, it is also not know with any certainty as to what happened to Enoch and Elijah after death. The bible says this about them -
And Enoch walked with God; and he was not, for God took him(Genesis 5:24).
Then it happened, as they continued on and talked, that suddenly a chariot of fire appeared with horses of fire, and separated the two of them; and Elijah went up by a whirlwind into heaven (2 Kings 2:11).
It would seem that Moses, Elijah and Enoch did not go to heaven as the bible says only Jesus had been sent there after his death
No one has ascended to heaven but He who came down from heaven, that is, the Son of Man who is in heaven (John 3:13).
It has been speculated that these men may be part of God's plan to use two of them as the 'two witnesses' spoken of in Revelation.
So getting back to your discussion about the fight between Michael and Satan regarding the body of Moses my guess is that the WTBTS is unable to come up with any logical explanation without a total revision of it's understanding of death (the spirit/soul), heaven and other places of spiritual abode.