I do understand your point of view, but I don't think you can compare the two situations to each other. I dislike Hyesung's mom not only because she left him, but because she came crawling back when he got useful. And when Hyesung asks her why she didn't find him sooner, she immediately throws it back to him 'Why did you not look for me'. - she has done nothing but play the victim. And I find that very Narcissistic.
Tho, I do agree that Hyesung's dad is most worse, when he comes crawling back. But I do not find Hyesung's mothers behaviour any better, and think the author handled this whole situation with Hyesung's mom poorly.
I'll bet my ass on, that as soon as Hyesung does something that does not suit into her little perfect world. she would turn her back on him in an instant.
Oh wow, now I actually hope that that will happen! That could be an amazing plot, for Byul to finally see his ''Grandma's'' true side, and turn his back on her after hurting his mom. Oh please author, let this be your plan!!
I think Hyesung did not get mad that long to Dojin coz he realised that if the situation's reversed, he might do the same. Remember what he did when Dojin blocked all of his family members bec of him? He still loves his mom, he just couldn't forgive her for abandoning him. Coz if not, he wouldnt allow her to talk to him unlike his dad he really don't want to see him.
He might have also thought that normally, Dojin wouldn't care about seeing/looking out for Hyesung's mom coz he is the type of person who shuts off even his family if they force him to do something he doesn't want. But now, Dojin's changed. Being with him and having a family made him compassionate and kinder, so it would kinda be like understandable if he did that.