There’s not much to develop for the other couple. That’s why we don’t see much of them. Old Xian already concluded the dynamic of their relationship. Their feelings are mutual but in a very asexual way. So, there’s not much she can do about that couple to keep things entertaining. That’s why Mo and He Tian gained so much popularity and have become the main couple throughout the years. Because their relationship is also physical and more complex.
Yes. Zheng Xi confessed early in the story. When Juan Yi was about to lose it because he couldn’t hold his feelings for him anymore. But Xi told him that love doesn’t have to be uniquely romantic or physical. And their dynamic was settled back then.
They love each other. But that’s it. Until this very day (future chapters with time skip) they have never crossed that line.
Also, I doubt Old Xian is worried about good writing. 19 days is like a lifestyle. It’s been out for so long and she’s never said she plans to end it. This is not a story that you choose to read because it has a collected and properly written plot. It’s more like a diary that you put random and scrambles thoughts about life in it. And it happens that these four characters are part of it. But good or bad writing? That’s irrelevant in this story.
I thought we would finally see more of the other couple, hell, I thought we’ll be focusing on qiu’s past and on how he met he tian’s brother, yet we’re back on the side couple (my as well be the main).