> Also, I wonder if Putin is really willing to continue this invasion at the cost of being ex-communicated by the US and parts of Western Europe? I suppose he is currently banking that most of Western Europe depends on his natural gas, so they cannot completely shut him out.
Yes, he is. This is as much about internal Russian politics as any international issues. The Russian economy is doing well, but most of the gains are concentrated in the hands of the ruling oligarchy. So the people haven't seen much benefit, and are angry about their economic situation. Putin has put out a lot of nationalist propaganda blaming NATO (specifically the EU and USA) for any economic hardship the people face. This distracts from the fact that he and his buddies are basically robbing the country blind. He has cast himself as the "defender of Russia" in a geopolitical battle against the US and EU.
A large part of this nationalist campaign has focused on building a "new Russian empire" from the former Soviet bloc countries. They've used the auspices of protecting "native Russians" in these countries (even though the idea of a "native Russian" is laughably absurd since the USSR forced everyone to learn Russian anyway) by providing Russian passports to anyone who wants one. They use the threat of invasion to extort cooperation from these countries and keep them in the Russian sphere of influence. Ukraine overthrew its Russian-puppet government, which prompted the whole invasion of Crimea.
Putin knows that the EU needs his oil, so he's testing his limits to see how much he can get away with. The EU has shown no signs of imposing any real sanctions because if they did, the EU would go through a similar situation that the US did in the 70s. So Russia's infractions have to go so far that the EU is willing to plunge itself into recession. I don't know where the limit is, but it's not anywhere close to where we are now.