You're not giving up, are you? Lol
for a very simple reason, I find it hard to see a future for Sumin and Sungji.. but then I can totally understand Seju and Sumin and see their attraction on different levels.. it also feels right given the so many uncovered hints.
I've noticed tho that the writer is a scorpio and shares the same birthday as Seju.. and all three represent the typical characteristics of the horoscope they have.. Sumin is so much of Cancer.. who is so attached to her mother (before she died) and Sungji is a Taurus (shares Gaji's sign)
I understand why they made that, it actually gives a guide line to design characters.
saying that, Seju's is in need with Sumin nurturing, and Sumin may find it very hard to find someone who will share her the same connection she has with the Scorpio Seju (she will fight till the end as stubborn with intense feeling and emotion that others will not suspect)