This all comes down to NEUTRAL restrictions.
When not at war in the Pacific, Russia must abide by the Nuetral Restrictions.
If a state of war does not exsist between Russia and Japan, then on the U.S. turn, NO the Americans cannot enter the Russia territory, as per the Neutral restrictions.
However, IF Japan were to attack Russia on it’s turn and make a DOW, then YES on the U.S. Turn they could enter. Or if Russia Attacked Japan.
As for the 12 IPC’s, you can’t Declare war on someone when it’s not your turn IE to block naval moves or something, or to allow allied units to land in your territories. So unless Japan makes a DOW on Russia, the USA is SOL, and no one gets the 12 IPC’s. If the Russians make the DOW JUST to allow American planes to land on the next American turn, Japan then gets the 12 IPC’s
3. Yes.