It's pretty pronounced in German like that. When I first read it I pronounced it hel-MEEN, which I don't like. It reminded me of Hellman's, a mayonnaise brand. I'd say Helmine is a legit name. It's a short form, which isn't the same as a diminuative.
When I say it out loud it sounds like I'm saying Hermine in an obnixious Chinese accent. But maybe that is because I just got back from the cinema watching Harry Potter. It seems weird to me.
Helmine looks incomplete to me. That's strange, because I can see Mina as a formal name before I could see Helmine. LoveMina. I do love Hermine ("hehr-MEE-neh") though. What do you think of that one?