Hey, I just wanted to say that your comment makes so much sense that it hurts haha :D I agree almost with everything, but I think you judge Donghae bit too hard here. It is true that Donghae is more on oblivious site most of the time, but it also true that Donghae is a open book, and he's always been like this - it's one of the reasons why they broke up in the first place, because Donghae couldn't control his feelings. And so, it's easier for Hyukjae to read him , because he has everything written on his face. And Hyukjae is a very careful person; he does so much for Donghae, like trying to be his best friend and support him, not showing his jealousy if he feels it, but it's not like Donghae's hurting him with anything. He doesn't talk about Andy or try to force Hyukjae to like Andy or anything like that. He even distanced himself from Hyukjae after a final break up, and Hyukjae was the one who approached him again, showing him that he is ready for this friendship again.

And also there is this one scene in the elevator when Donghae feels awful for his after-sex state and that Hyukjae can see it. Doesn't it show that he is aware of this weird tension on Andy (or dating next person in general) subject? He even feels stupid to think that Hyukjae cares, because Hyukjae doesn't seem to care... Or like this one moment when he was buying rings and they jumped to conclusions that they are for Andy. He wanted to explain, but he didn't have a chance, since Hyukjae gives him no reason to.
You know, I think that Donghae grew up so much through the story and those messy decisions he make are not always bad. In the end, I don't think Hyukjae would love him if he was more like Hyukjae himself; if he was any different than he already is.