What do you think would have happened if Jon left the Wall and joined up with Robb and his army?

I think it's hard to predict because so many different things could happen! First of all I think Robb would be shocked since he believed Jon had left for the wall! second I'm certain Catelyn wouldn't like it at all! But I don't think by any chance that Robb would execute Jon without question! Robb is too loyal for that! He would ignore the rules if his brother came to fight by his side!

Robb and Catelyn would also have quarrels whether it's right or wrong