If Roderick Blackwood survives and is crowned King of the Riverlands with the support of Arlan III what might happen next? Would Roderick use the opportunity of the Brackens having lost the war and drastically reduce House Bracken’s lands or even exile the House and thus end the B-B feud? What would his religious policy be after the quasi-religious war just fought? Does he have the legitimacy that the Teagues lacked and thus the opportunity to strengthen the monarchy vis-à-vis its vassals?

Interesting question!

While Roderick Blackwood would certainly have a number of advantages that Arlan III lacked, I don’t think it would have been smooth sailing. 

For one thing, despite the fact that his side has won, they’re pretty battered too and the Brackens aren’t going to go gently into that good night. So the new Blackwood dynasty would be starting out with a long bloody siege of its own vassals.

For another, the Blackwoods are in the religious minority, ruling over a kingdom that’s staunchly of the Faith of the Seven. And the Faith Militant is over the border in the Reach and unlikely to give up the ghost against a heathen usurper oppressing the faithful of the Riverlands.

And finally, the Blackwoods would have gotten themselves onto the throne with help from a foreign power. Arlan III is going to demand his price in blood and treasure just as in OTL, and that’s going to grate – how long before the Blackwoods are seen as quislings?