As long as it’s not blocking Russian ships from the Black Sea the US can do anything and not much would come out of it straight away. Some countries would try to become independend of the canals but that would likely not lead to war.
On the other hand, the US literally shooting at Russian or Chinese ships would be a declaration of war that would require an immediate response of those attacked else it would just escalate to them losing more ships and probably even their ports and coastal infraestructure/cities being targeted soon as well.
Anti-ship missiles, cruise missiles, drones and many other solutions would make it not sustainable for long enough to hurt these countries, specially if Russia nuked whoever gave support to such privateers as their new nuclear doctrine (that they presented in the last 48 hours) would likely require they answer like that.
That’s why veto power exists, and if Russia lost their veto power then Russia would not follow anything comming out of there and the UN itself would likely go the way of the League of Nations.
Two can play that game. Although I don’t think there are any chokepoints for that anyway.
If civilian ships got hit by the “exercises” that would be an act of war because to the victim that is just a direct attack on them.
That’s the same as invading and taking Russian land and would likely be considered an act of war and nukes might fly. And there is also many ways to avoid crossing into the wrong borders anyway so it shouldn’t be a problem.
Again, the US can circle China as much as they want but they can’t fully block them without attacking China/Russia which would be considered an act of war.
It would be good but those lines are still attackable by drones and missiles, that’s why there is no way for the US to block them without nukes flying being a very high likelyhood.