This is a very interesting topic. I have a couple questions concerning it, though I doubt there is any specific answer.

1) If the orcs are corrupted elves, then could the very corruption which changed them from elf to orc have altered them enough to take away their immortality?

2) Could this very question be why Tolkien decided later that the orcs were not corrupted elves? If orcs are human, then they will live a normal lifespan. In addition, they will multipy much faster than elves do. Both issues would seem to be relevant to Tolkien deciding that orcs were not elves.

Of course, since the canon of Tolkien's work describes orcs as coming from elves, in my opinion that is the true nature of the orcs. What Tolkien may have intended to change matters not, for he did not change it. Christopher is not his father, and while he may have all his father's notes, he still cannot see inside his father's head and understand all that was going on there.