You know how marriage works right? overall on these matters, the lands may "legally" belong to his wife but you never get a word from her, it's clear as water that ha can manage the lands as he pleases, not to mention the children (if they have) would likely inherit both the lands and the Orlesian titles.
I did just want to point out that a large part of why we get away with this initial landgrab is that DuRellion has a questionable ability to enforce the claims. In order to do anything about this, they'd need to go before the Crown, which would need to confirm the treaty that his wife claims entitles her to the land. That he's an Orlesian will make this claim harder to pursue, since Ferelden and Orlais are still not getting along. Some people are so touchy about spending a century as a province. So, to say that it's clear as water that he can manage the land as he pleases is a bit of an overstatement.
Legally it might be, depending on what exactly the treaty says and what Ferelden's exact policies are on ancient legal documents regarding land with limited settlers. So if that's what you meant by it being "clear as water" that he can do what he wants with that land you might be right. De facto? Not so much. So, if anyone's wondering why Ferelden's not doing anything about the Inquisition basically taking Haven in a bloodless coup, it's probably because they don't sympathize with the man they're (probably) supposed to do so for. Compare this to their reaction when the Venatori take Redcliffe and the monarch(s) themself(ves) ride in personally to toss Fiona out.
And as to your actual point you're entirely right. Orlais doesn't control that territory. A man who owes allegiance to the Orlesian Throne does. (Or more accurately, he is for all we know legally entitled to.) And thanks to his wife's claims in Ferelden (ie the exact territory we're discussing,) he also owes allegiance to the Throne of Ferelden. My understanding is that weirdness like this was not atypical back in the day. Only there's no point in telling TKS any of this, since he never changes his mind on anything whether or not he's actually correct.
(In case it's not clear, about half the point of this is a rebuttal to TKS. Which, I admit, is probably pointless.)