This issue just came up in an email.
Mana vs. manna: I figured either word works for what I wanted to convey. I believe Larry Niven, within the fantasy genre, was the first writer to use the first word to mean magical energy, but I think the term actually comes from the magical practices of some culture that I can't recall. The second specifically refers to food given by God, but metaphorically refers to something that is there for the taking,
In the end, it's just a handle for the stuff that powers magic. I wanted to be clear that it is different stuff from everyday physical energy and doesn't necessarily follow our laws of physics. As far as I'm concerned, it can be called anything. That's why I had Ursus say that it has lots of names and we might just as well call it manna.
Of course, now I have people thinking I'm making a homonym mistake. I guess I should have just made up a word.