[Morsul the Dark]

Lastly, we now that the Balrogs had become incarnate in their forms and so coud not change their forms to escape from falling, but what about Sauron? In the Sil it says that he changed into a vampire and flew away after his match with Huan. So why didn't he change into a flying creature during the destruction of Numenore and escape?

I believe this has more to do with the fact, and I have no quotes ready so bear with me. Although Middle-Earth is a fantasy world Tolkien kept it very realistic. The Mariner(name escapes me) who listened to these tales at the cottage of lost play was a man and the elves were in my opinion reprenting classical cultures Romans and Greeks and such who made these myths to explain nature and even those stories based on real people were blown out of proportion I believe this was part of Tolkien's layering, if you will, of history into these books the story passed on orally grows each time until eventually its is hard to explain why something happens one way in a story and another way in actuality