By this reasoning, at any point in the series that she felt like it, Kes could have demanded her lung back.
Even if Tuvix had been allowed to stay unified, Kes could have still demanded her lung back because she had given it to sweet lovable Neelix and not that creepy asshat Tuvix.
It's just a question of how badly Tuvix's lungs would have been mutilated as they were minced through a seive to extract the Ocampan DNA.
Only assholes get caught regifting.
Neelix had an Ocmapan lung grafted onto his body so he would have been still 100 percent Talaxian (Scientific Method, I know.) everywhere else within the parts he was born with... But Tuvix's birth would have consumed that lung on a genetic level to the point afterwards that a fraction of every cell in his body was a little bit Ocampan.
Maybe the reason Kes was revolted by Tuvix was not that he was a dick but becuase he was herself. Tuvix at his core was a shitpoor clone of Kes herself and even as dodgy as it got in Fury where they tried to runaway together, Kes did not want to make love to herself