Would it be stupid to believe that JK won't automatically assume the worst of Kim Dan? He's shown that his personality is completely flawed, and he's selfish. Excuse my word vomit;
But I think that might be just due to how intensely he hides his weaknesses, Kim Dan being one of them. My prediction is JK has caught feelings for Kim Dan, and is fiercely trying to hide/undo that fact by being a POS(which is unforgivable). Paired with his anger issues and personality, this makes him especially dislikeable and selfish.
Moving on, he's made it clear he sees Kim Dan as one of his people, because of that whole arc, I think it might be possible that he becomes the slightest bit aware and realizes what's actually going on. Maybe even getting defensive on Kim Dan's behalf. Alternatively, this would reveal that Grandma is deteriorating more rapidly, and maybe then he'll have the mind to show some respect and empathy.
This is expecting a lot of JK, who's shown himself to be somewhat of a complete narcissist, I know. But I imagine this author will try to redeem JK to some extent by showcasing some form of loyalty.
I also reckon Grandma's favor will become relevant to their dynamic somehow, and that JK has actually read Kim Dan's letter/card.
I think it's important to note that this series focus was never romance, and a BL tag or not doesn't erase the queerness in the characters. This is relevant to multiple series, not just this one. Either way the story and characters are lovable, and it's worth the read.
I actually forgot this had a BL tag -- I felt zero romantic tension. And honestly it doesn't particularly detract from the core of the story, which has always been about the horror mystery elements. I'm sure that the story rather than romance (if any) was what kept readers going this whole time.