Ah! I’m actually glad you commented on this. I had done the “extended spine” thing on purpose. Most people write it off as a mistake, but when you think about it, it actually makes more sense. Take this image, for instance.
Obviously there is some funky stuff going on there with her spine. Because rib cages cannot bend, all of that flexibility she’s showing in these images is largely influenced by the fact that she has a segment of her spine missing. We aren’t going to get into the physiology of how she still walks because I really cant help you there. Comics. That’s the best answer I can give you.
Anyway, If you look at the picture up there, I’m staying true to the whole “rib cages can’t really bend” thing. I still wanted to find a way to push the limits here and find a way for her to go through herself.
Literally what’s happening here is that she has stretched the flesh between where her spine ends and begins again in her abdomen to make way for her head and shoulders. The same amount of space that the hole is open, her spine has separated. It’s like if you had two separate plastic bars end-to-end in the same rubber tube. You can still stretch the rubber tube where the plastic bars aren’t attached, but the ends are still solid so they won’t stretch. In this case, her rib-cage is one of those plastic bars, as is her pelvis.
So in any of the cases where it seems like her back is “broken”- it literally is. In the cases where it seems like her spine has separated from itself to stretch unnaturally- it literally has.
Oh- and my explanation on how she still keeps herself upright is by corset. (Fun fact- there are a few humans alive today who have worn corsets so tightly for so long, they can no longer stand upright without them! Their muscles have deteriorated too much to support their own weight That’s where I got the idea that Kanaya would use them because she COULDN’T stand upright by herself if she had wanted to.))