Also the last season closed with an explanation of why such a war is impractical—you can only send through people who are dead in the other reality. Depending on how advanced Generalplan Ost is ITTL, the Soviets might pull that off...or their soldiers still have, at best, a 50/50 chance of getting through without melting.
No, that door is closed, I think.
A cave in the middle of Pennsylvania is also a very bad place for Soviet armies to deploy.