So if Robb hadn’t essentially gotten manipulated by Jeyne’s mother and Tywin from afar into marrying Jeyne, how do you think Walder Frey’s ultimate betrayal and murder of Robb and his cohorts would have gone down instead?

I would imagine it would be a gentler Red Wedding indoors, with the noble guests thrown in chains rather than being murdered, although the outdoors part would probably be unchanged b/c Walder would need Robb’s army destroyed so that they don’t put the Twins under siege.