Assume I have loaded 2 UK inf on a US transport and then in the US turn I moved the transport to the seazone next to Ukraine. Can the inf on the transport participate on an attack on Ukraine in the following UK turn together with other UK units? Does it make any difference if Ukraine is empty and the units can take it alone?
The infantry can participate in an attack on Ukraine.
You do not have to unload both infantry during combat, although you may if you wish.
It makes no difference as to whether or not you unload infantry if Ukraine is empty or not.
Except that you are forgetting that Japan will have around $40 IPC aimed at Russia also while the UK and US safely bomb Germany and rely on Russia to fight the ground war. Which would work… except for those 14 divisions per turn coming at Moscow from the east…