I've always liked the idea of Infested Kerrigan as an undying being who even if her body exploded into thousand pieces she would be spawned back into life by her own will. Like a self apart from the human Kerrigan.
I would hope that'd be covered in "Heart of the Swarm" or associated materials (like something equivalent to the manga depicting Raynor's return to Mar Sara just before the beginning of SC2).
It'd be a pretty big issue and if everyone in the know about Sarah Kerrigan and the Queen of Blades being the same person (most of Raynor's Raiders, more than likely, although the Dominion public as a whole seems unaware) is entirely cool with her hanging around, it wouldn't be really realistic.
Well, I believe that Sarah is sort of trapped inside the Queen of Blades. In the final level of the campaign, near the end, you send Kerrigan back underground and the Queen says "You shall pay for this treachery", and then it shows Sarah in her human form, saying "Don't give up." So...