If Robb had declared for Stannis before being crowned, how would Stannis have rewarded him, longterm, assuming they crush the Lannisters? And if Stannis pushed his religion on the north, how would Robb have reacted? (I would assume he wouldn’t accept it).

Well, the timing doesn’t work there – Robb was acclaimed before Stannis put himself forward as a candidate – but…

I think Stannis would have rewarded Robb by A. punishing the Lannisters (because they’re lawbreakers), B. returning Robb’s sisters, family sword, and father’s body (because it’s the right thing to do) and C. recognized him in his ancestral titles (because that’s the law). 

I don’t know if he’d be inclined to do more than that, because as Stannis sees it, Robb owes Stannis his allegiance as a matter of law, and you don’t get brownie points for obeying the law. Now, as a matter of practical politics, a Stannis who needs Northern swords to carry him to the Iron Throne knows that you also need to reward service in proportion, so if Robb does a good job as he’s likely to do, I think Stannis would be willing to grant a request he saw as within reason. 

No, Robb is not going to accept R’hllorism as a state religion in the North. But in this scenario, I don’t see Stannis feeling that indebted to Melisandre, so…