Contradictions. Oh, yes, this is the downside of having published collections of author's notes, differing versions, ideas, etc. You wind up with so much information, inevitably, some of it is going to contradict -- especially when you have the author admittedly attempting to try to make what he has already written fit into a specific belief system that can't quite manage to hold all the extant details. What happens to a "dead" Maia? (I believe Tolkien did say that Melian shed her physical form and returned to Lorien, so we at least know what happened to her.) Not just Curumo and Sauron, but others like the Balrogs and other servants of Melkor. Tolkien never really says. I know that he tried to come up with some sort of explanation for what happened to the spirits of dead orcs, but never managed to come up with one that he could reconcile both what he believed as a Catholic and what he wanted the orcs to be.
One also has to consider that when we read these stories, we are reading things written from the point of view of a Hobbit or an Elf. They do not know the details of what happens to a fallen Maia, and thus can only speculate. In the case of the Istari, however, we do have one indication of who decides their fate when their real physical bodies died. Upon the death of his body, Gandalf/Olorin went "outside of time and thought," which Tolkien says in one of his letters meant that he left the Circles of the World; his fate was decided by Eru. Eru may have other fates in store for the other fallen Maiar. Since Curumo's body died within the physical world, and the physical world is so much more than just Middle-earth, he may be doomed to a wandering existence that will take him far away from it. I believe it was said that Sauron had so diminished, he would never be able to reembody, and would be nothing more than a shadow of fear and evil in the world. Boy, I think I need to go back and brush up on some of my reading!
