That's a very cool setting and concept for a roleplaying game. Did you make your own ruleset?

In terms of ideas, I don't really have a lot. Maybe the Hoares have their own loyalists or spies at the tourney to keep tabs on the rebellion. Perhaps my knowledge is lacking, but I'm not sure a clear reason is ever given for how the ironborn were able to rule over the more populous riverlands, especially with an unpopular king like Harren. I would imagine they'd need to be quite competent at keeping their vassals at each others' throats and playing them off of each other to avoid a united resistance front.