If Russia invades Ukraine, will NATO send troops to fight Russians? If no, what's holding Russians back now?
Absence of power. Russians gathered about 40K most combat-ready troops on Ukrainian borders. It is a lot, but by the word of military experts absolutely not enough to occupy a quite large country with population of 40 millions. They can enter and occupy some areas of Ukraine, they can probably even march to Kiev and take it, but than what?
And no, NATO will not send troops to fight Russian because as it means a nuclear warfare.
And to the topic: No, WW3 is not coming. For WW3 to happen Russia has to have some allies, but the Russia is absolutely alone. It can be World vs Russia, but unlikely.
BTW, those, who thinks about China & Russia. Chinese troop are specifically trained and geared-up to fight... Russian forces. China has an eye on Russian territories, which are now populated (illegally) by Chinese. And by taking the Crimea Russian showed an example... Dangerous example.
And to the question: "What the hell is going on in Russia?"
My personal take on it...
Putin is tired to play democracy, even pretend to play. Because it does not guarantee absolute personal power. Chinese governing model does not guarantee absolute PERSONAL power. But North Korean - does. Putin building new North Korea, where the main component of the regime - isolation for the outside world for the country's populations. Putin does everything to isolate his country form influence of the outside world. Look at his every step, even today, when he banned import of food and closed Russian aerospace for Ukrainian flights. He has built very efficient and powerful propaganda machine, and while his support among Russians is at all times high he can cut-off all links with the world and enjoy absolute personal power without any influence from anywhere. Pretty much like in North Korea.