> If the EU decides to join the US the war is over and Russia will keep the occupied lands.

As a European, I'd say there is just about 0 chance of the EU unilaterally supporting Russian taken any occupied areas to themselves and Ukraine surrendering. Not only would it signal to Russia that they can take European land without consequences, but public opinion is very much against any sort of cessation of defenses. In my ~30 years I've never seen as strong NATO support from the common man in countries like Sweden and Spain as there is today.