The best use of those Russian units is to get 3 IPC for China and allowing China to place units in Manchuria (hopefully with artillery).
Doing this creates problems for Japan as a large stack of Inf covering a growing Chinese Army is problematic.
Granted this doesn’t always work out the way you want. However, as Russia I am always waiting for the opportunity to take Manchuria so that China can stack an army there.
I don’t particularly care for the race back to Moscow with those units, because it basically gives Japan the green light to ignore the northern Chinese front which means the DEI and India face more Japanese units. Even worse is when you’re halfway back to Moscow and all of a sudden Japanese armor just sucked nearly 5 IPC / round of territories from you and you have to decide to stop the bleeding and make a stand or keep retreating to Moscow.
I still think that Russia should have the option in something like Round 6 to move half the 18 Inf via “train” to the Western Front, forfeit the Mongolian event and lose the other half of those Inf.