Well the rumors have been stating that they have one or two doubles that they use in addition to Lupita's motion capture performance, so the arm may very well be real with the face CG'd. However, I wouldn't be surprised if the arm was CGI though, since there is the technology to do a shot like that. I mean Gollum in The Hobbit looked amazing, if only the other CG in those movies looked as good.

I have little doubt it's Maz Kanata handing the lightsaber to Leia, though I guess it could just be some other alien.

Also on the weird theforce.net forums, there was a fully organized compilation of all the rumors/speculation into a full plot, mostly from Making Star Wars stuff. Who knows how much is legitimate, I'm betting maybe 20% of it is correct at most.