Regarding your posts, here is what I think about some questions still not answered. Tell me if you agree with me.

First, I think the real father of Seju is the "8/10" man. Seju's mom had an affair with him. He seems to be the same that came to visit Seju (same mole) in the name of her actual father.
He cares about her even it's in a psycho way. He even gave her her name. Strange.
He shut his mouth when the family says Seju's mother was a whore.
It could explain why Seju's father doesn't come to see her and doesn't seem to involve himself because he knows.
It might come out if he dies as his health seems to be weak.

Then, the glasses girl seems to be very threatening.
It's obvious she took photos of Sumin x Sunji. She just noticed a strange atmosphere before.
But she told Choigun though they were in a relationship. How could she know? The author gives us hints of the bad outcomes if somebody knows in the office. It's not only because of the past of Sumin.
I think she will spread the information because she's jealous, frustrated that Choigun doesn't look at her.
When it will happen, we will have a flashback of when it happens in the past for Sumin and how she dealt with it.

I think Sunji's parents are dead. She never talked about them. We never saw them.
She lives with her brother and sister in law. We saw her friends...
When she was young, her brother was the one who consolated her when she was sad about her schoolmates.
Furthermore, she seemed to know what to tell and to manage when Sumin talked about the death of her own parents.
Sunji seems to be immature but she's not. She's just innocent and pure but smart.
Some people assume that when you're too sensitive you're weak. It's not true.
On the contrary, you're stronger than others because you are hurt quite often and you keep moving forward.
She shut her mouth but she understands Sumin a lot, she even knows what's in her mind because she's very caring.
She's full of surprises and not an empty shell. In my mind, even if she's younger than Sumin, she's the one who will teach her a lot of things. She's not bitter (by the way she doesn't like bitter things) of anything and she faces what she lives trying to keep smiling.

Then, I think Sumin loves her from the beginning. It was love at first sight too. Remember the scene of the cherry blossoms. "Badump, badump". She kept asking herself what was this feeling. Nami feels it too by speaking with Sumin "it's not like you will give her your heart" and telling Seju it was a risky date. She used to be lonely. It's moving for her. She doesn't know what to do when good things happen to her. She was jealous as well from the beginning.

You are not this much confused and scared if you don't feel anything. She just not realized it yet or not live with it.
Sunji is a good thing for her. She starts smiling again. You don't try to be better if you don't love the person.
She stops smoking, drinking. She's faithful. She seems happier. It's part of the thing.
And you're not afraid of loosing somebody if you don't love them.
She cares for Sunji and she's trying to avoid her pain. If it's not love I don't know what it is.
But I agree something is missing between both of them. It's not the sex thing or maybe how it will happen will be the trigger. They say they like each other, even Sunji says she loves Sumin but they didn't show it really. We don't really see complicity.
Maybe because they were not confronted to real difficulties yet. Maybe the coming out of their relationship at work will do it.
Sumin is holding back because she knows what will happen when she will stop doing it. When she will open herself we will find what is missing. But strangely Sunji knows more about her than Sumin. I mean, the heart.
Sumin is ashamed of her past, that's why I think she's not ready yet to talk about it with Sunji.
First because she doesn't want Sunji to be splashed by that and then because she's afraid Sunji will see her in a different way and reject her.
Now she trusts Sunji so I think it will happen soon. She's more and more sweet with her.
There is not so much gap between them. They first experience a true relationship. It's a first for Sumin too.
The problem is that it seems to be hard for her to accept to let go memories with Seju. When she's with Seju she thinks about Sunji and when she's with Sunji she thinks about memories with Seju. Not the Seju of today, memories. She's a spectator of her life, always chased by her memories.
Since she began with Sunji she barely spoke with Seju and became actress of her life.

It's a healing story it's said. We heal by receiving/giving love, forgiveness and understanding... So...

I have a lot more to say but it will be in another post. Why Seju hires Sunji... Nami and Seju...
Seju and Sumin... The third wish (I think Sunji will surpass herself)...