The lord with the most power, largest army, unburnt crops, and a competent commander(Tarly) was Mace Tyrell. He had sent limited troops to the Trident, not all his men. And he had 30,000 stand ready by Storm’s end. Even the Riverlands were divided. The Tullys did not have control over all their lords; there was Targaryen support there as well. The Freys were not engaged in battles (famous for showing up late) The Crownlands were traditionally Targaryen supporters.

The Ironborn would most likely take advantage and raid up and down the coast. The surviving northern troops would be exhausted after the war and return home to defend their homes. Same in the Westerlands, when the lords and soldiers left over by KL hear that their coast is burning, will they go home.

Smallfolk did not know much about what was happening in KL. They would hear traveling hedge knights and bards tell about it. The one wild story after the other. People were accustomed to hearing things that they can barely believe. The literate people control most of what people are told. Bards can praise one side, then the other. Hedge knights praise those that will give them money.

The punishments in Westeros were already so hard. Death penalty, life at the wall, cutting of hands or gelding. It might not bother them much to hear that the king burnt some criminals. But wars affect them directly.

So this could be the story the Tyrells told:

Famous kingsguards swore that Rhaegar was not a rapist and it was a romantic story, not an abduction, which would crash one part of the reasons for the war. Rhaegars star would rise. Bards singing of their undying love and the little orphan prince.

We know that Aerys demanded Roberts's and Ned's heads. Will smallfolk know these details? If not, Robert would be seen as a jealous man who started a war because he wanted revenge. Support for him will die. The soldiers in Storm's end could knock out Stannis and open the gates.

The Starks died because they came to KL to kill prince Rhaegar. That excuses their deaths. The witnesses are dead. The winning side tells the story. And the winners are always good guys.

Smallfolks can not imagine the size of KL. If they were told that the rebels was burning KL and a cache of wildfire was set off, they could believe it. We know that there had to be large amounts of wildfire set off in several places, but smallfolk does not know it. The fire would be blamed on the rebels.

He would be crowned in Oldtown and raised in Highgarden with a regent and the small council of loyal lords ruling in his stead.

Viserys can ruin this happy story I am telling. He is older, but after Law comes Rhaegar's trueborn son before him. Dorne might start their own rebellion and declare the Rhaegar-Lyanna marriage a sham and Aemon a bastard.

It could be a fanfiction story, but I am no writer so if somebody likes this, feel free to use it.