I keep forgetting that Mizore isn't pure white. Because the manga is greyscale, and she's made of snow and ice, it's easy to overlook the fact that her skin is a normal pinkish colour. It gives a different nuance to all the mutilation she undergoes.
So they're fine with using her cut up, broken apart body, but not with cutting her stomach open (which has probably burst open from at least one of these body scramblings)?
It could also be like one of the problems with vampire immortality in UQ Holder I suppose. What was it, as long as the pieces are separated and never destroyed they can't reattach to each other or regenerate from nothing? Hellish way to be trapped for all eternity.