Well Grace is with Ewa, and her Avatar body may not have decomposed yet. In the first movie, she said something like “It’s a global network, and the Na’vi can access it. They can upload and download data, memories, at sites like the one you just destroyed”. I’m thinking her memories are with Ewa and will somehow be put into her Avatar. Who knows, maybe someone as important to the Avatar program as Grace gets more than one Avatar in the event one gets killed. Maybe we’ll see a new Grace avatar, with less of the human features, and its body will have her memories downloaded to it by Ewa. Anything can be made up for this really. For example, jake could have some super Na'vi brain, being a mix of the human brain and all, and be able to do things the other Na'vi can't. Maybe he has the ability to extract Grace's thoughts through the Tree of Souls, save them in his brain, connect to Grace's avatar, and then transfer her memories to the body that way. A human element might be needed to tranfer the consciousness. Mo'at did say "her wounds were too great, t'was not enough time. She is with Ewa now". Well, Jake's brain and body are perfectly healty, maybe he can do for Grace what her human body was unable to do.
As for Quaritch, I don’t know what to expect. Bringing him back is going to be challenging without making it seem forced and unrealistic. It’s safe to say that the humans have mastered cloning, especially seeing as they were able to grow an organic life form while mixing DNA from two species that evolved light years apart. I see one of three possible scenarios. 1) Quaritch has a cloned backup body, exactly like the one that was killed, and just has his most recent memories dumped to it. 2) Quaritch, in the first film, was actually an avatar. He controlled himself without actually putting himself in harm’s way. The real Quaritch is somewhere else. 3) They somehow bring him back, and in conjunction with one of the first two things I mentioned, end up getting him into a Na’vi looking avatar in an attempt to pull off what Jake failed to do. I’m hoping they don’t just throw him back into the mix though. Maybe begin to brew that batch in the second movie, and then have him come back full villain in the third movie. Who knows, maybe he’ll just be in the prequel.
Sidebar. It's a stretch, but I'd love to see footage of Earth where they have captured Na'vi on display at a zoo, or something like that.