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Yes i agree the body do control a lot of fhe actions on human body, but this is not something the human body controls. this justu is controled by the byakugou seal. And the byakugou seal is matined by chakra. regeneration is forced by this. even though this is medically impossible to surive, she surives it because it is forced. That is why the justu have such heavy drawbacks.
the reason she summoned katsuyu is not because of herself but because of the kages sake. If she just needed to heal herself she would have used regeneration. medical ninjutsu consumes to much chakra as it is regeneration is on another scale. The kages were her first concern so regeneration was out of the answer.
this pretty much sums it up.
Let's talk Byakugou seal. Tsunade had to direct the chakra into the seal and then activate the jutsu...once activated it heals her without a need for hand seals/mystic palm. The problem then becomes this, the Byakugo rests on her head.
This mean if she does get an automatic heal from it, it would have to reform her ENTIRE body. As she's stated the body can only divide the cells so many times...decapitation would virtually be suicide to reform. Additionally she'd have to remake organs, and everything through through simply chakra alone (with cells in her head). And even then it's more likely the jutsu activates on command
As we saw, Tsunade played medic to heal the Kage so she did not heal her legs....while she may not even be able to..let's assume she could heal her legs by reforming them. This is far more difficult with a brain due to the fact its decapitation. If Tsunade could remain concious, her brain would be acting on overload and panic having just lost its body (chicken with its head cut off-like). She'd have to forcefully want too and focus on reforming her body.
But what opponent is going to sit there and watch?! They'd start curb stomping her face into the ground. And that's on top of if she'd even have the cell count to reform her whole body.
Which is once again why I firmly believe a swift and precise decapitation is fatal.