How much do you think the Whent marriage contributed to solving the traditional Riverlands issues with overmighty vassals, atleast for Hosters generation? And also, separate question, but because the Whents have been essentially wiped out by this point, is Edmure now also the rightful heir to Harrenhal?

Maybe somewhat, although Harrenhal doesn’t seem to have been a major destabilizing force since the reign of Maekar I.

That second question is a bit tricky. Under normal inheritance rules, you’re quite right. But Harrenhal is special and hasn’t obeyed normal inheritance rules repeatedly over the last three hundred years. So it probably depends on whether the Iron Throne is exerting control (as they are with the Holy Hundred) or whether the locals are in charge.