If Yohn Royce’s faction had overpowered Lysa and entered the war early on Robb’s side, how do you think they would have reacted on the North and Riverland secession from the Iron Throne? Would they have joined the new kingdom, left Robb or continued to fight for him as an ally?

Interesting question. If they’re already at war with the Lannisters, they can’t recognize Joffrey, and they’re kind of in the same dilemma that the North and the Riverlands were in at the end of AGOT.

Given the history between the North and the Vale, they’re probably not joining the Kingdom of the North without some really compelling reason – maybe a royal marriage between King Robert Arryn and Princess Sansa/Arya – but even then it’s iffy. They might continue as an independent ally, but that’s difficult politically.

OTHO, it’s quite possible that if the Vale is there in the council of war that the King in the North moment doesn’t happen. The Vale is one of the most consistently loyalist forces in Westeros – Robert’s Rebellion aside – so they might call for Renly (the smartest political move at the time), Stannis (the legal move at the time), or for a new Great Council to decide the succession.