If she wanted to write a toxic drama, she should have taken inspiration from Flowerchild or other mangaka well verse in that type. Here, nothing makes sense, to the point it's funny. Which is a bit disturbing somehow for a seasoned mangaka..
Interesting that you think so because I think everything makes complete sense and Ohsawa Yayoi knows exactly the type of story that she wants to write here. I think so far the writing is very tight and every chapter is significant to the plot. She's pacing out the mystery of the mess that is Hana extremely well and I don't think she's trying to make any character here "rootable" or "likeable".
This is also too much for me to make the character believable. I feel like she is just a device plot to keep the story going.
It you can make sense of those characters, well, you can try to light me up. But we are already past the half point of the story, and we have yet to see Hana's point of view, or seing more about their past that would explain Hikaru's obsession with her.
About the husband, saying that he wasn't written to ben likeable or relatable while tons of people are rooting for the het couple on a yuri forum, that's definitely not very honest...
I think that is where we differ in how we see and enjoy the story. You're trying to relate or see realism in these characters, but I'm just seeing these characters as concepts (or as you said, plot devices). Hana and Hikaru are extremely interesting to me because they are on extreme opposite ends in character personalities and it's interesting to see where they will lead the story. They're fun because they're not realistic and relatable.
What's engaging about these characters is the "why". Like why is Hikaru so obsessed with this girl who is 5 years older than her since she was 5 years old? Why is Hana obviously stringing along Hikaru who she knows has been obsessed with her since childhood?
I don't think the author is necessarily making the husband "likeable". He's just the concept of the "loyal, faithful partner" to counter the "dangerous, exciting homewrecker" and those traits are what a lot of people find "likeable". Frankly I find him pretty boring and it's obvious to everyone (even himself) that he comes 2nd in Hikaru's heart. The story is just to see what concept Hikaru is gonna choose in the end.