Team Blonde here. Been lurking in this thread for a month, but you guys tend to move the thread so quickly, it's hard to join in.
I'll pitch in my 2 cents here on the latest chapter, focusing more on Sungji's presence in the chapter since it seems like you've all analyzed Seju and Sumin pretty thoroughly.
When Sungji finally appeared with her "Team Leader, right now, the one next to you is me" and her hand outstretched, I viewed it as Sumin being aware of Sungji's want to help Sumin out (via their relationship), even if Sumin doesn't seem to reciprocate or accept that help.
Immediately after that panel though, comes redhead seducing Sungji and saying that Sungji's too good for Sumin. Since Sumin seems to be analyzing her relationships and doing some introspection of herself at that point, I think it's safe to say that Sumin believes Sungji's too good for her, and that she fears Sungji will leave her for another woman, since Sumin's a "garbage-like person" and "an empty shell."
What hurts my heart is that (at least, this is how I interpreted it) Sumin responds negatively when asked if "there's someone who'd hold an empty shell like you," and that someone was, I believe, Sungji. Sumin recognizes Sungji's presence in her life and her infatuation with Sumin that's a lot more refreshing and carefree than her relationship with Seju, yet Sumin still doesn't seem to want Sungji to "heal" her.
It's possible Sumin's just being a depressed piece of shit since her mind's breaking down at the possibility of no longer having Seju and she's not thinking straight, hence her clouded view of Sungji, but alas, this story has become so unpredictable, it's hard to even test the waters ahead since everything's so ambiguous.