1) She calls herself a "devil" as a metaphor for her going against the will of "god" (Madoka). However, she has phenomenal cosmic powers either way, so it really makes no difference.
2) No. The "Madoka" we see at the end of the film is one of two fragments; the part of her that is a real-world being. The other part - the "Law of Cycles", as they call it - is isolated away from her. Homura ripped the two parts away from eachother, but if they were ever to reconnect, Madoka could erase everything that Homura did.
3) She doesn't declare Madoka her enemy; she says, effectively, that it is inevitable that they will become enemies, because of their very different goals. Homura, inarguably, loves Madoka - but she feels that Madoka was blind in her own selflessness, and only made the choice to become "god" because of her circumstances at the time. It's often interpreted that Madoka's universe was born from selfless love, and Homura's was born from selfish love - but how selfish is it to create a universe in which everyone, yourself included, can be happy? Homura does everything she does for Madoka's sake - and I'm sure that if Madoka saw it in that way, that they could never be enemies.
4) No, again; they are being kept at bay by Homura's power. If they're removed entirely, then universal entropy would continue - which is bad. They basically serve the same function as they did in Madoka's universe, except they have no control over the magical girls anymore; Homura got mad with them for screwing with Madoka's universe, so she can't risk that happening again. They're still alive, but subordinates to Homura.
5) We don't know. The fact that Sayaka's wish remains true despite her still being alive is puzzling, as is the scene in which Homura apparently "wipes" her memory of the previous universes. It's up for debate whether or not there are even still magical girls in the new universe.
6) To manipulate Godoka, and restore the "Witch" system, in order to gain more energy to stave off universal entropy. Battling Wraiths gets them enough energy, but Kyubey is greedy and wants more.
7) I certainly hope so. My prediction is that, due to contact with the Wraiths, some of Madoka's power will be restored, cuing a battle between Homura and Madoka - who will then have to settle their differences in order to beat the Wraiths (who will probably be more powerful as a result of there being fewer magical girls to balance them out). Following that, there are two possible outcomes; firstly, that they beat the Wraiths, and Madoka will choose to live in Homura's world despite her having regained her powers, so that the two can be together. Alternatively, both Madoka and Homura die beating the Wraiths. That's just my 100% fanon prediction, though, and I'm likely to be wrong. ;P
