Then what could stop Putin from just using the ukrainian military mobilisation as a casus belli and conquering Ukraine like it's middle ages all over again? Or would that be too extreme even for him?
I don't really think anybody could or would stop him if he tried that (except of course the Ukrainians themselves, but they wouldn't have much of a chance). I just doubt he would try that because it's an extremely bad idea.
He would gain Ukraine, but it would be destabilized, impoverished economically and would probably have a resistance movement going on. With the Russian army presence he strengthens his enemies in Ukraine already, as the threat of war always strenghtens nationalists and the more extreme people on the political spectrum.
Also he would isolate Russia internationally, it would be like the Cold War all over again. I don't know what he's up to, but I think he is smarter than that. He will try something to keep Ukraine in close ties with Russia, because he thinks he is losing Ukraine to the West. It would be more useful to him if he somehow manages that peacefully.
So, I hope at least, maybe this is all sabre-rattling, to show that Russia still wants a say in Ukraine. That's not so great either, but at least it's not a war.
BTW the G7 have cancelled the G8 meeting in Sochi now. So while there are still talks going on, that's a first taste of isolation.