I'm probably about to be absolutely lambasted for this take, but I'mma say it anyways.
I've always been of the of the mind that Qingheng-jun and Madam Lan's relationship is more fraught with complexities than it seems. All we know is what Lan Xichen told Wei Wuxian, and he didn't even want to know why his mother killed his father's teacher. What else doesn't he know that he therefore cannot tell the audience?
If she did not wish to be with Qingheng-jun, why did she even agree to marry him? How could she have been forced? Why did she later even listen to the elders? She had enough ability to kill at least one man, arguably a very strong cultivator himself, what reason could she have had to have walked into the cage Qingheng-jun opened?
Ultimately, we only know that she might have come to love him rather than that she never ever did. The text is still fuzzy on what came before their marriage and subsequent isolations. I think the popular/widely accepted interpretation might be putting undue limitations to her abilities and who she could have been as a person.
And, if you're curious, my interpretation/supposition of what came before is this:
Madam Lan, [insert preferred real name here], was a very powerful cultivator. She met Qingheng-jun and rejected his romantic advances but accepted his friendship; hence why she was still anywhere near Cloud Recesses. His teacher later committed some crime either against an innocent or someone she knew. Nobody listened to or dared to even bring forth their accusations because the teacher was a Strong and Well-Respected Man in a High Position (TM), and therefore she went to confront him herself.
They argued, they fought, she won, he died. Qingheng-jun offered to marry her to spare her his clan's wrath (or to protect his clan from her blade if she was powerful enough). She agreed for [insert reason of choice here], and they were wed. He, obviously, had mixed feelings about all of this and locked himself away. She fell in love with him at some point. At some point they had Lan Xichen and then Lan Wangji. And then some years after Wangji was born, she died. How did she die? Murder by poison, maybe? And Qingheng-jun later died after being grievously injured during the Burning of Cloud Recesses.
Yes, I understand that my interpretation is generous to both Qingheng-jun and Madam Lan. Yes, I do understand that my interpretation leaves little room for either of them to be absolutely horrendous and evil people. For all I know, Teacher Lan just looked at Madam Lan the wrong way one day and she murdered him in cold blood ala Xue Yang. For all I know, Qingheng-jun lobbied those murder accusations after the "accidental" death of his teacher, forced her to marry him for her protection, and then locked himself away in pure guilt (he orchestrated his own teacher's death and used it as a trap); but not guilty enough to let her go.
