Without a binding constitution, there is no formal legal basis for a polity and people are free to do as they wish. E.g., join the Russian Federation or constitute themselves as an independent republic.
You're starting from the fact that Russia is more powerful than Ukraine and working backwards to find justifications for genocide.
Has Russia made changes to its constitution illegally? That would be news to me. There is of course a strong argument that the dissolution of the Soviet Union in 1991 was not constitutional based on the results of the referendum.