Two of your proposed engagements would echo the iran-iraq conflict for body count because they will be contested on sea and land, and at great cost to the aggressor. The last time Korea was fought over approximately 5m troops were committed. I guess that soaks up a lot of china's unemployed youth but I doubt they will be happy about it.
There are no cheap victories here. Once the more modern stocks of weapons runs out, it's slings and arrows.
Henry Kissinger will fix it. Don't worry.
Russia doesn't want to rebuild the Soviet Union. They want to rebuild the Russian Empire. The Baltic countries, Finland, and (all of) Ukraine were parts of the Russian Empire and are more likely to be invaded that ex-USSR states like Armenia and Azerbaijan.
Putin sees himself as Peter the Great and not a "dirty little Georgian peasant" like Stalin.