I'm not too worried about Donna - they explicitly said that the gizmo affects all humans, Donna is not affected therefore Donna isn't entirely human; she's remembering, she's going to die, but she'll regenerate.
Really wondering what happened to pregnant women etc. suddenly transformed into the Master; when the magic reset button is pressed will their embryos magically reappear? What about the ginormous paradox all this represents? And so forth.
If Wilf kills the Master will all of the replicants regenerate as different Time Lords?