Thanks a bunch for responding to my Renly succession inquiry. More or less matched my own thoughts on the matter but your confirmation of them was most welcome. Had a followup question that I meant to ask at the time: What impact might Renly’s death at the tourney have on the brewing conflict in Westeros? How does the Wot5K change as a result (aside from there being only four kings)? For the purposes of discussion, let us assume Robert grants the Stormlands to Tommen.

Good question. To begin with, the Tyrells don’t really have a dog in the fight and are likely to hold back and try to play kingmaker with the various claimants. 

Tommen, as a mere child, a half-Lannister, and a rumored half-Lannister, would get no support from the Stormlands. Chances are, when Stannis raises his banners, a good chunk of the Stormlands rise with him. Not all of them, hell not all of them rose for Renly, but a good deal of them.

Basically, Stannis would have been able to get a roughly equivalent size army to the one he got in ACOK much faster, and chances are would not have had to worry about the Tyrells slamming into his flank at Blackwater – while the Lannisters might have tried to make a deal with Mace for Margaery’s hand, the armies of the south not being mobilized and certainly not being mobilized that close to King’s Landing would have made their arrival come much much later than OTL.

So most likely the Battle of Blackwater would have happened weeks earlier, without the boom chain. Stannis’ navy would have taken a good deal of damage, but he would have been able to cross with Salladhor Saan’s ships and force his way into the city with his army largely intact.