I think he should have brought House Frey closer, maybe helped Walder make some advantageous marriages. Just so that way he could keep an eye on them. After their whole “being late” during Robert’s Rebellion, I think for any Lord that should be an issue. A Northern Lord would have most likely at least come with an a strong force and ask what happened, if not outright hang you for an oath breaker.
the Riverlands are a quarrelsome bunch. I would have used Harrenhall as a far carrot for all the lords to be loyal. Especially because House Whent wasn’t going to outlive it’s current Lady, who was already elderly. And that would mean the best claimants to Harrenhall would be Hoster’s own Son or Daughters. And well, he had much higher aspirations for them, so Harrenhall was a nothing gift that would have empowered a House to be very much loyal to him.