Hm, I had forgotten about Demetra. I like it about the same as
Demetria - I like its 3-syllable-ness (instead of 4). And it doesn't share the drawback (the one drawback) of
Demetria, which is that it sounds vaguely like a medical term. People would probably misread Demetra as
Demetria sometimes. I think Demitra is fine too; or even Dimitra is intuitive, because of the name
Dimitri.
I'm so used to seeing
Demetria, though, that I think I'd feel safest with that. And, since I'm not all that comfy with four syllable personal names, and I'm not all that inhibited from certain kinds of name-sinning, I'd probably call her some much shorter nickname like
Deitra, Mita, or
Dima.
I think
Demeter seems cool, but I'd have to cross it off my personal list since I don't even know how to pronounce it properly.