"How tf is he supposed to know" I mean, that's the thing, isn't it? You just said it. He never knows jacksh!t about her and only makes assumptions. Their entire relationship started from his assumption of her which led to an obsession. He never tries to actually learn about her.
And this *is* dark romance. The way you described the story is alright but that does enter in the dark romance category. The thing is, it's just not made very well so far. They sure got the messy and dark drama aspects of it, but the ML itself is simply unlikable so far. You asked for a different way in which they could've gotten closer at this point? It would've been him going to the frost lines, despite the risks of his status which would make him an even bigger target than the rest, to see her and the life she wanted to create for herself with his own eyes. Then at some point he gets injured during the battle and in a typical cliche, she sees him, gets scared for him (because we do see she still has feelings) and takes care of him. From then on, with or without sprinkles of amnesia, they see each other in their vulnerable states and learn how to comunicate, etc. Again, it is cliche, but then again the way the actual story is written is cliche too so why couldn't the author choose a cliche that makes the ML a better person instead of a selfish fk I don't know.