1. The USA is a NATO member nation. 2. No they won't stop the Russians, but the Russian invasion will hardly be a surprise to NATO after they've invaded these countries. 3. Most of the neutral European countries likely won't have a choice but to fight. They certainly didn't in the last two wars. 4. No I don't think anyone has the capability to hack the satellite intelligence capabilities of all NATO nations, or even one of them. 5. China will do what is best for China. Exactly what that is I don't know, but going to war with its two largest markets is not it, I think.
Your edit: Yes I think the Russians are desperate to show off their new capabilities and thus remain relevant in world politics. That's why they're showing off their newest prototype fighter. That's why this thread exists.
NATO tanks are consistently heavier than Russian tanks. Russian tanks are at or around 40 tons. NATO tanks are at or around 60 tons. A 50% weight advantage.
I'm sure the Chinese can mobilize far more than 3-5 million men if they wanted to. I wouldn't think 100 million men under arms would be impossible for them if they made the effort. However, quality is again an issue, and how would 100 million men move half-way around the world to Europe? Walk across the Eurasian landmass? If China decided to support a Russian invasion the war would be over before they got there. And when they do get there they would be under constant interdiction from NATO air power, to say nothing of their supply lines.
Composite armor is used on the following NATO and European tanks:
US' M1 Abrams
UK's Challenger I and II
Germany's Leopard II. Other NATO nations fielding the Leo II: Canada, Denmark, Greece, Netherlands, Norway, Poland, Portugal, Spain, and Turkey. In addition these neutral European countries operate the Leo II: Austria, Finland, Sweden, and Switzerland.
France's Leclerc
Serbia, Croatia and Slovenia's M-84
Italy's Ariete