If Robb doesn’t marry Jeyne Westerling is there still a Red Wedding? I’m not trying to avoid the RW here, to the contrary, I’m trying to find some other things that could cause it or an event of similar magnitude.

According to GRRM:

“…knowing old Lord Walder’s character, it is likely he would have searched for some way to disentangle himself from a losing cause sooner or later, but his desertion would likely have taken a less savage form.”

So probably you’d get a scenario similar to the RW, except that the guests are captured rather than killed.