She knew very well she did a mistake, she felt bad for it, but still walked out of her bed and worked to get back to Sungji. In the past Seju was unable to do so, she had to be dragged out from her bed by her bro
I think you're mixing things up. Seju was not dragged by her bro to get out of bed, Yes, he did go see her, but that was after Seju had already broken up with Sumin. And, Seju did drag her ass to explain things to Sumin. They even had a fight on the floor or whatever that is called.
Well, Sumin was once in great pain and had to visit a hospital, remember? Doc seemed to know her very well. She's also tired very often. Whatever happened to her, it wasn't just the cutting for sure
The doctor did remind Sumin about her lifestyle, that she should be careful with it, so I don't think the cutting has anything to do with her back pain. But it is possible that she has an illness nobody is aware of. That's if that scene does have a meaning and not only a filler scene.
Not so magically. And the weren't even fully naked when Sungji found them together in one bed. Sumin found Seju after she had sex with that bartender, Sungji found Sumin before things went too far. This is one of the differences. The other one is that Sumin actually explained why it almost happened and worked hard to earn forgiveness while Seju never did such thing.
I don't think there's any difference. Cheating is cheating. Sumin had the choice to not cheat on Sungji, but she did it anyway. Whatever her reason is, she cheated on Sungji. And you are wrong, Seju did try to explain things to Sumin but Sumin just wouldn't forgive her even after 10 years has passed. While Sungji, she forgave Sumin only after a little more than a week, after she tried hooking up with Dawoon, and because she was horny as fuck. That's where the difference is.
One of the reason she's doing so, leeching on Seju, in my opinion, is to get back on Seju for everything. She might see it as a compensation for what has happened to her, what she believe was caused by Seju. Because Sumin once returned everything Seju gave to her and then moved away from her house. She also found a job at this company on her own, although at that time it was a small and poor company (before Seju bought it). So, yes, Sumin can live on her own, not leeching away from Seju, if she wants to. But I might be wrong.