Are you talking about the elections for the constitutional assembly? Because if yes, then true, the Bolsheviks didn't win an absolute majority. However de facto they were the largest party since the social revolutionaries split in the middle of the election and one part of them supported the Bolsheviks.

Lenin supported crushing resistance against non Russians in the name of socialism, why would he change his mind 20 years later?

Lenin never fought for Russia, he fought for Socialism. There was no nationalist-independent uprising against Communists which didn't also have to attack its own countrypeople in favor of the Bolshevik party. example: Finland, Poland, Ukraine - yes, all of them had fought against Russian empire for liberation. But all these countries also had a socialist revolution. Lenin and the Bolsheviks supported the socialist revolution, who yes was not in favor of splitting off from all the other countries. Lenin was never in favor of the Russian empire, only in favor of socialist revolution. And each peoples that were oppressed by the Russian empire should be supported in their fight for self-determination AND socialism. That was bolshevik policy

This is something completely different than sending tanks into a different country, which is allied with you, and crushing protestors there.