I have no idea, but I find it really interesting that a) a Crow Tribe child apparently has enough autonomy to be allowed to choose where to stay on her own, and b) her tribe is apparently fine with her interest in humans.
I guess that if she can/could fly, she could've flapped off and not returned? Birds don't seem to be very interested in keeping prisoners, AFAIK.

She looks really ill, so I definitely assumed an illness. And I think it had to be something incurable. If modern health care could have actually helped, I think Ya Qing would have made sure she got it. (And Ya Qing seems pretty familiar with the modern human world.) Perhaps it's a Crow-specific illness, but the Crow Tribe doesn't have the resources for research into a cure?
Nodnod. I wonder whether it'd be something specific to Crow Yashou, or a bird disease? If the latter, it might explain why Dong Nan spends most of her time in human form. And yeah, the Crow Tribe doesn't have the resources for research.

(I haven't found any fanworks either, but I did find this Chinese article about Dong Nan's actress, Wu Yujue, which has a pic of her with Bai Yu.)
Oo, ty! I guess they were shooting all/most of the graveyard scenes at the same time, then? I don't think they shared a scene.