I could see Hamilton going if the outcome of the FIA investigation isn't productive, IE. removing the idiotic overriding "race director has control of the safety car" clause. I don't see why Massi going or staying would be a pre-requisite of Merc dropping their appeal. Massi could stay and the issue be resolved, or he could go and exactly the same thing could happen again. It's a procedural issue more than anything.
No chance whatsoever that Rosberg will come back. I'd put more money Hulkenberg than Niko. But as above, Merc would just buy Occon or Bottas out of their current contracts.