The legal mechanism to split up Russia is to decapitate it, so that individual regions will cease cooperation with Moscow.
This is nonsense. Decapitation is not a legal mechanism, it's WW3.
The individual regions lack the capacity to function without Moscow. The state borders were designed by USSR planners to make it extremely expensive (high likelihood of war) to break apart. The borders intentionally cross ethnic boundaries to make a future breakup worse than just tolerating the borders.
Decapitation by NATO would be a war. But it’s fine if it’s done by someone from inside Russia - or perhaps by unnamed “terrorists”.
Regarding Moscow - can you explain what does it provide that regions need? And how does it make hard to break apart when it’s already been done in the past?
The UN Charter can only be amended by unanimous agreement of the Security Council. So Russia can't be removed from the Charter. The Charter could effectively be dissolved, and a new membership could agree that Russia needs to be split up, therby declaring war on them. But Russia, China, and likely Indian out of such a system, how legal is that really? How legal is international law that only 1/2 the world agrees on?
We've moving past the stage of ever making things right with Ukraine, or Russia. I don't think any of the outcomes look good. Nuclear proliferation will start again. And absent institutions designed to avoid conflict.