Regarding referendums, there is an interesting post with some historical data confirming that the countries of the so-called collective West (led by the US and UK) are ready to recognize or not recognize the results of referendums, only when they get benefits from it.
Seems fake. USSR constitution allowed secession and did not specify any requirements (like a referendum). So technically member states could secede in any manner lawful in said states.
Article 72. Each Union Republic shall retain the right freely to secede from the USSR.
That's it. That's the whole article on the subject. It was predicated on the obvious (at the time) fact that no one in their right mind would attempt to coerce any of the republics to secede and the article was just for show, not unlike human rights guarantees and other parts of the constitution.
AFAIK Yugoslavia had something similar.