So to summarize

oml, finally a person who uses their brain and actually reads the story T.T

I don't think most people expect him to already, thoroughly and compassionately apologise for everything he's done that's hurt Dan. However, if someone needs to be told that being treated like a sex doll with complete disregard to one party's wellbeing - EVEN IF you have a sexual agreement in place - then that's on Jaekyung. He doesn't need to know that Dan developed deeper feelings and every single reason that led to him leaving, also Jaekyung isn't responsible for everything going wrong and burdening Dan in his life, but what he IS responsible for is being a decent human being and trying to understand the other person and have respectful discussions with them, especially when they are visibly having a very difficult time. So no, that's not too much to expect from someone, it's just that Jaekyung is THAT far up in his own ass to care about anything other than his own interests up until now.

Yeah but than again, JK doesn't know he's being a piece of shit. He haven't come to the realization yet. He knows he went overboard with Dan after losing the fight, but doesn't know how much that hurt Dan, he knows he wants Kim Dan back but doesn't know exactly why (he is just blaming on the jinx). He thought that their relationship based on sex, physical therapy sessions and money was what they both wanted because him and Kim Dan lack communication skills. He doesn't understand that he was an ass with Dan all this time, this last chapter was the first time Kim Dan vocalized that he was uncomfortable with JK there. He can't begin to understand Dan and show empathy if he doesn't understand himself first, first he must see what he did wrong, that he didn't treat Kim Dan like a person, let alone a lover.

I'm not arguing with you about him not exactly understanding how his behaviour affects others, but it's not because he can't understand (unless he's literally a psychopath). It's because he doesn't want to understand, doesn't want the responsibilities of even minimally caring for anyone other than himself, not even when the guy he supposedly needs for his livelihood to get back on track almost died of exhaustion and depression.

General social clues alone should've been enough to shake him shake him into offering up a superficial apology even. But that didn't happen, so now suddenly changing because Dan isn't being swayed by his money anymore is underwhelming and disconnected , don't you think? I find it to be a character development hole, the pacing and the order is off with what he experienced vs his reaction. It went like "Dude don't fucking die yet , not before I get my career back up" and we're supposed to feel that he's gonna show actual regret later on bc Dan isn't responding to be fucked or treat him as he used to?
Showing empathy to someone doesn't only depend on the other person telling them exactly how they feel, Jaekyung would have to care in the first place to ASK. Which he's never done.
So yeah, it's not unreasonable for many of us to have expected a run of the mill apology at the very least, so that there's cohesion with the events and shows potential for any real future growth and redemption to take place with Jaekyung.

I think JK is a lot like a kid who needs someone to point out his mistakes like "You fucked up big time here buddy" cause he lacks the social skills to acknowledge that otherwise. Everybody walks on eggshells around him and puts up with his shit, except for coach who beats him and screams in his face that he is wrong, which works.
Don't get me wrong, I want him to apologize too, if it was me dealing with him irl I would have kicked him in the nuts long time ago. But expecting him to suddenly grow a conscience and empathize with Kim Dan without truly understanding that he is a piece of shit is not realistic at all.
It will be a few chapters before he sees that he was wrong, he doesn't deserve Kim Dan and then apologize from the bottom of his heart.