I just think that Mokyeon Got Nanjo and in his life he got to to know something deeper. Before that he used to.live a pointless and aimless life. I feel.like For Nanjo in some way Sunhwa had an importance. Maybe in his mundane life he caused some tide too. Not necessarily romantic but something different. So being snatched that option away without his own choice or even decision might have irked him. Also I don't think Nanjo has stronger feelings for M yet. Sunhwa is equally partly responsible for us third party but for him he was the first link of this weird triangle relationship. Idk it's just my view.
That's one way to look at it. Despite, being lied to and used. Maybe like what you said, sunhwa did make his life a little less boring and that gives some meaning to his mundane life and perhaps, he want to cling to that a little longer.
I agree with many here. wtf is with nanjo? Not about how he treated mokyeon but towards sunhwa. He doesn't seems to like sunhwa that way but yet, he treasure him?? Even after he know the truth from mokyeon?? Whatever survivor complex he has towards him should be over now since he know sunhwa is no victim. I don't get him.