I wanted to see if I could figure out what the point of divergence might be, that has happened in the background, and I have got a theory.
From what you gathered, Victarion is in charge in the Westerlands. Though that campaign likewise overachieved. Instead of only burning the Lannister fleet like in canon, they sacked Lannisport and briefly even attacked Casterly Rock itself. Whatever they are doing though didn't seem to work as well there though and they had to retreat.
The point of divergence is further back than the start of the rebellion.
We are told that the point of divergence is from before, the Ironborn rebelled
There has been a fairly significant change in he background that is only now starting to impact the trajectory of politics.
I don't think anyone picked up on what it is so far though.
Based on this quote, then it seems that the change has been hinted at in the text, so I decided to read the first post again and look at the text about the attack on Seagard.
A few memories became more focused at his words. You remembered being near the docks at the time when ships approached silently. Black banners with a golden squid. There was a terrible roar like a dragon and a bright flash. Screams. Fire. Yellow flames mingling with green as men swarmed from the ships. The sound of stone crumbling coming from the castle at the same time as the Bronze Bell sounded for the first time you had ever heard it.
I think that there is a lot of interesting elements in this quote, but the most interesting part is "
Yellow flames mingling with green as men swarmed from the ships.", since when I read that, then it sounds like there is both yellow and green flames. If there actually were green flames at Seagard, then it is most likely the sign of the use of wildfire, though I do not know how the Ironborn would have gotten their hands on it.
Two other interesting parts of the quote are "There was a terrible roar like a dragon and a bright flash" and "The sound of stone crumbling coming from the castle", since it seemed like that the Ironborn had some way of throwing something destructive at the castle. The thing that is thrown is likely wildfire.
On the same day as Seagard had been sacked, the same had happened to Lannisport in the Westerlands. The entire Lannister fleet had been put to the torch and there were persistent rumors that the Ironborn had tried to use some sort of sorcery to attack a keep called Casterly Rock, though ultimately had to give up these attempts. They had seized the Fair Isle since then and were raiding along the western coast of the Seven Kingdoms, all the way down to the Reach, with little to no opposition.
The Twins had fallen too, which elicited gasps from most of the room and even surprised you. You had talked with people who had been there a few times and you knew that it was deemed very difficult to siege the double keep as one had to do so from both sides of the river.
However, Euron Greyjoy had allegedly used sorcery to badly damage one of those keeps and killing a number of people from the household, including the heir. Lord Frey surrendered after threats to have the keep razed entirely and his entire family clasped in irons to toil in the mines of the Iron Isles.
We can also read that the Ironborn are said to have been using some sort of sorcery to attack both Casterly Rock and the Twins, which is likely the same type of attack used against the castle at Seagard. Since people are calling it sorcery, then it likely mean that it is not a common tool at sieges, that are being used, which I think would support the idea of it being wildfire.
The choice was taken from you a moment later. All of the sudden, one of the boats was on fire and in the shine of the flames, you saw two Ironborn hurl burning clay jars at other boats.
Someone yelled to get the boats away, but by then you already were trying to loosen the knot of the rope. It didn't move at all. Seeing more jars being hurled, you fumbled for a knife that laid in the boat, just cutting it. Luckily, no one seemed to have the time to attack you and with a push of the oar, your boat drifted out into the river and away from the flames.
When we were attacked by Ironborn, while traveling to Riverrun, we can read, that they were throwing burning clay jars in an attempt to burn boats. I did not find anything talking about the color of the flames in this episode, so while these clay jars might not contain wildfire, it does suggest in my opinion, that the Ironborn has alchemists giving them weaponry.
So, in summary, I believe that the Ironborn has gotten their hands on wildfire and figured out how to use it against castles and boats, which has caused the Ironborn Rebellion to be more successful, that it was in the books.
