Cybermen isn't so difficult to explain, there were cybermen in the prime universe before 10 crossed the void into Lumikverse allowing those cybermen to follow him back - That doesn't explain how the Prime cybermen are built to Lumik's design though, maybe Lumik's cybermen sent out a message, maybe the Prime cybermen were monitoring earth when Lumik's cybermen invaded and they updated themselves to the superior design. They wouldn't get pulled back through the void, they didn't have any contact with voidstuff.