1. Love the idea, that should be great fun. xD
2. Great, and of course, I'd even suggest she can look like a variety of girls too. 
3. That sounds reasonable to me. :)
4. Ok, up to you.
5. So reading up on it, historically axes have been up to 6lbm in weight. Given how demons are stronger than humans in general, I'd say it'd be ok for her to comfortably wield a 10-12lbm. I'm trying to think of enchantments that could make something lighter for a wielder but nothing comes to mind from the top of my head. Do you insist that it's mass-based? I think it'd be easier for a sort of safety-lock spell to exist that allows only her to use the axe fullstop. And maybe the special thing about her is that such spells don't work on her in general as she can get "under the skin" of items as well as people? 

Donny is a-ok~