Logistically, How would the King of the Norf & Trident work out? If Robb Stark have won

[Frey Kings]

Both Kingdoms are vastly different and the only thing holding them together is the marriage pact (and their hatred for the Lannisters). Unless the Tullys and Starks continue to remarry, I don't see this union lasting long and may break apart in a couple generations. Some say the Riverlords need the Northern Army to repel the invaders. Thats fine. But it would take a while for Northerners to arrive and probably would be better if some part of the army just stay instead of going back and forth. Which could end up being positive and negative. or The wild card is the Vale. Who knows what Sweet Robin or Littlefinger or Harrold Hardyng or their descendants will do.

 

I could see the North itself achieving and maintaining independence but not the Riverlands. 

[Canon Claude]

You’re right. But the problem was that the Riverlands were in such a bad position that they hooked their wagon to the one kingdom that was helping them out. The Riverlands border almost every other region though, so sealing it off from attack is almost impossible.