Removing US troops is also a win for the USA - less costs. Also, China isn't going to invade a new Korea - they prefer keeping them as a vassal state
That, plus the carrot of Trump not committing to an all-out trade war will keep the Chinese happy
As to the Russians, or rather Putin, that's a mystery. Russia has far, far less influence over Nork than the Chinese do (similar to US versus UK influence over the Philippines), so Putin may have been told by Xi to shut up on this. Also perhaps Trump said "Ok, Ukraine remains in your sphere of influence, in exchange for this"
The Russian thin g is pure speculation on my part.
But here’s the catch: Stalin got Kim Il-sung to invade the south in the first place. He got Mao to go along, thanks to favors owed for support provided during the Chinese Civil War.
I tend to view Russia in this case as the wild card in the whole deck. Having said that, I also believe you’re on the right track when you say China would really, really, like the US out of the Korean peninsula.
One can only hope Mr. Trump’s dealmaking skills include plans for regional security after a formal peace treaty gets signed ending the Korean War.