How would the politics and War of the Five Kings change if Lord Walder Frey had died before the series started?

Interesting question. I suppose it depends on how quickly things go to hell in a handbasket – obviously, Black Walder is still going to make his move, but he has to kill his way past a lot of Freys. Ser Stevron is the new Lord and he’s got three sons and seven grandsons. 

Also depends on whether the Freys are there for the disaster beneath the walls of Riverrun, and if they are, whether they get away intact (as I think the Mallisters did). 

The first major difference would come with Robb’s arrival at the Twins – Stevron doesn’t have the sheer chutzpah to extort Robb, so he probably lets him cross without all the marriage stuff. Which means Robb’s hand is open for negotiations with the Iron Islands, Dorne, or Highgarden post-Renly, and there’s not the same excuse for the Red Wedding as in OTL.

However, for all that Stevron is a generally standup guy, he’s also generally very cautious when it comes to matters of grand strategy (he was the one saying that Robb should let the Baratheons and Lannisters fight it out and bend the knee to the winner), so he’d probably still pull out of Robb’s army if as in OTL the Tyrells allied with the Lannisters. However, it would be far more low-key and less murderously damaging than the Red Wedding.