“Russia’s terms, as reported by press, are a reasonable starting point”
Sure. I’ll be moving into your living room and taking over your swimming pool. Sounds reasonable to me.
You coming into “my” living room would be wrong because it is mine.
But, when it come to places, who is the owner? The principle of self-determination is that the people there are the owner (assuming you have an identifiable people and an identifiable place).
So, if the people of South Sudan are alienated from Sudan, they can form their own country. Ditto East Timor, Kosovo.
Sometimes, the parties involved are civilized, and can proceed peacefully, e.g., the Czech Rep. and Slovakia. But, usually not. Instead, because of some combination of economic interests and deep emotional attachments, there are wars.