I think they call it the "reality metric". Russia owns and occupies the land and makes the laws there as they have for the last 10 years. The citizens who live there overwhelmingly consider themselves Russian.
Political power grows from the barrel of a gun, so to speak.
Yeah, exactly. Those who didn't consider themselves Russians have been purged long ago, forcibly deported and driven out from their lands since the early Soviet times.
Tatars and Ukrainians have been targets of ethnic cleansing since a long time in Crimea, and therefore Russia should be granted the land as a prize. Just like with Koenigsberg Kaliningrad, it's very easy, if you kill and deport everyone in an area, then that area is yours.
Whether they consider themselves Russian is a slippery slope of an argument --and not only because I doubt you have trustworthy sources to back this claim. There are many regions in the world where minorities or even significant local majorities consider themselves separate nations.
If your argument that political power grows from the barrel of the gun is to be followed, then there's no problem with Ukraine using said power to take over Crimea, should they have that capability. That kind of follows from your own logic.