(Tumblr glitched so I am resending in case it did not the first time). If a second son was given Harrenhal, would it be feasible for him to tear it down and build a more practical castle?

Well, yeah, it’s feasible. A fair bit of “Dark Age” construction was done by re-purposing dressed stone – which is an expensive and difficult to manufacture and transport materiel – from Roman infrastructure. 

But whether that’s enough to dodge the curse, I don’t know.