Didn't Walder say he has 10x more men than Blackfish? I would assume it's almost impossible the twins would fall to blackfish.
Sansa is supposed to not trust Littlefinger, Maybe he has Bolton men at Moat Cailin.
I think you're getting ahead of things there. Sansa was told her uncle controls Riverrun but she doesn't trust LF. She sent a trusted knight to verify that information and gather his support, that's it.
We know from the books that he can't possibly help her, and that the reason Brienne was sent there is a different one. I wonder if anyone in the other thread made the connection between Brienne and Jaime both going to the same place.