Is highlighting Relath's position as a viceroy in front of him a good idea? Sure he knows it's the case, but I think he mostly views it as a formality for placating the mortals. That's technically the case here, but I think he knows enough now to realize Viserys actually gives a shit about his mother's opinions.-[] Explain that while many of the current laws regarding Succession will be grandfathered in due to Westeros not being as malleable to legislation regarding that subject as the East, and the East having a few stand-out cases mostly in regards to underdeveloped regions that would not be productive without generational guidance regardless, that by and large you plan on rewarding different vassals not with larger grants or land, but instead pieces that you can shuffle around through the long ages.
-[] Further, explain that Relath is a viceroy, and while he holds to his oaths and provides just rule to those within his remit, that is not a position likely to ever shift. And if it did, it would be due to Relath's willing (cue scoff) resignation or death beyond any and all recall.
-[] So quite frankly if she (Wavespinner) is only seeking to place part of her clutch on the Throne of Tolos, she will likely leave quite disappointed. And it's so far from a concern even a long-lived sorcerer would have about a dragon liable to live thousands of years so long as his overlord holds him in favor that it wouldn't be worth scheming over anyway.
We don't want Relath thinking too strongly on the "technicality" that Viserys can just revoke his ownership over what he effectively views as part of his hoard. That's the sort of thing that rubs a dragon the wrong way, and leads young ones into doing stupid things.
She also suffered greatly as a result of effectively the same logic. If there was ever a topic she'd reevaluate her cultural preconceptions on its this one.None of these people married Relath willingly... at best they grew to accept, maybe even fall into with a will, their circumstances.
Viserys might feel compassion for them in that, but he wouldn't exactly worry over something that was arranged in the first place not to place them in positions of any great influence other than "hostage to your good behavior". Their happiness is also largely secondary to the stability of Tolos if she's thinking in terms of someone raised in a feudal environment. Many women who's husbands die in a couple of decades despite being much older in Relath's case have even less personal freedom even after the fact than these women do at present thanks to his laws.