Yes, turning off background romances should be possible as well. I even think it's a mistake to pair off two of the most popular romance options together, but fine, whatever, just don't force me to ship the same characters the devs do, as you said. Especially since his flirting with Neve is better than his flirting with Rook for some reason, from what I've seen. In fact, he feels like a completely different person romancing Neve or Rook, like they didn't know what character they were going for. Fickle, as you said. And if the devs wanted them to be together so bad, they shouldn't even have made them romance options, especially Lucanis who they clearly don't want us to romance.
From a narrative POV, just why? Is it Spite? Show me. Is it because of his family/the Crows? Show me. Nothing explains any of it, and if it it did, it would still make a better narrative to explore a relationship with him having made a hard choice he obviously disagrees with, like with Neve, than locking him out (and it wouldn't explain why he romances her afterwards anyway). From a meta POV, just WHY? Three men to romance and one is the only romance who can be locked out, romances someone else after (someone, again, who made the same decision that locked you out) and for some reason has more chemistry with them than with Rook. Why? It's lazy, inconsistent and weirdly cruel to the player. And, of course, it happens with one of the male romance options and NO ONE ELSE.
Worst of all, I think even if they see this and care a little, I don't think it can change much. They could lock his romance with Neve if he's locked out for you, I guess, and that would be the bare minimum, but it doesn't fix everything else. They won't add a romance with him post saving Minrathous now because they would have to write, voice record and everything, and I just know Bioware as it is now is not going to do that for a patch. They knew what they were doing when they made these decisions for I think the most popular first romance for women, and they don't care. We get to be lazy on that one, right? Bioware doesn't respect it's fans, so I'm done respecting them.