Laws are usually there for a reason.

You could apply your whole argument to public opinion and referendums - public will change mind a lot, why make such drastic moves based on it?

ex-SSRs can be a good example but ex colonies isn't a fair comparison with Catalonia

Yes, often quite a bit of violence in the run up. Whether the imperialist side wants to try and look good at the point of capitulation or not has little to do with how “legal” the effectual parts of fighting for independence are.

To suggest that, e.g. Gandhi didn’t have a meaningful role in achieving India’s independence, because the British considered his actions illegal, is very silly.

Saying that India got her independence legally is quite a stretch. There were plenty of mutinies, bloodshed, violent and non-violent movements before the British (massively weakened by the WWII) decided that they had enough. While it is true that India did not declare unilaterally declare independence (some leaders did, but they did not gain widespread public support) and start a civil war, the independence movement itself was not legal and most leaders were branded terrorists and incarcerated.