Is there a fandom term for the novel's ancient setting, like YOHE for the drama, do you know?

The Primordial Past is what I've seen commonly and what I myself use.

And also a very mundane example of being hopelessly besotted

Yes! And it really drives home that they live together and share these mundane everyday things.

I think they could have made it work for the drama, even with the very different relationships there!

They made so many things work, I'm sure it would have been delightful. I love Chu Shuzhi's relationship with Shen Wei so much in the drama.

drama!SW would never be in this situation in the first place, for so many reasons

Yes to all of this!