You completly paint here the MC as the one at fault yet you forgetting the other real ending when the MC ignores that his mom is banging bully and the Mom becomes irritated with her son and starts compering him to bully! She ain't as innocent as you portrait her! Bully raped her and she let it slide!
Pretty sure that in your ending the mom yust found it convienient that he left and the bully definetly gaslighted her to hate the son (hopefully)
PS: " And I don't really know if the mother and bully relationship in the end is genuine love " bruh! The bully literly forced her son out of the house! If the mom lets that slide too then she's not a "mother" but warden!
The MC should have showed some resistance when he caught them red handed but he just stood there like a fool and watched, even Aiko tried to resist goichi but ultimately succumbed to pleasure while the MC stood there watching.
And not to mention he ran away with out looking back and didn't even visit or call his mother for years, not even once.
So it's obvious she would go back to the bully. And even if their relationship is not a romantic type, there's still some kind of bond between them and not to mention the bully has a history of aborting pregnancies and and having many girlfriends and using girls for his own desire and breaking up with them but I don't really blame him since just like most NTR mangas the antagonist became like that due to lose of their family in the bully case he lost both his parents at the age of 13 according to what he told Aiko in the early dialogue and that might be what shaped his personality.
But him chosing to stay with Aiko and keep the pregnancy shows he changed in some way cus his old self had used and dump so many girls and girls also used and dump him so him staying with Aiko for that long and wanting to start family with her proves that he sees Aiko no longer as a conquest but something more and considering the romantic pregnancy settings and scenes at the end, it proves they should have some kind of bond or affection mixed with lust. So it's not total affection or love and it's not total lust either. And he didn't trick her into taking her company which he showed signs of doing earlier in the story. So both the bully and the mother changed and have some kind of bond or affection between each other but it's also mixed with lust.
As for the MC, he screwed up by leaving Aiko for years with out even reaching out to her, and even if she became harsh in the other endings she still wouldn't have the mind to abandon him, but he abandoned her and ended up in a fucked up life showing regret for abandoning her instead of trying to help her overcome her problem, of course she would move on with someone who was possibly there for her cus I'm pretty sure she would have been heart broken after Keiji left and the bully was the only one she could really on for emotional support and possibly they developed some kind of affectionate or possessive bond mixed with lust especially the bully.
So the MC is 80-90% at fault for his situation.
And yes the bully was a scumbag but he became calmer and more affectionate towards the mother at the ending even if their relationship was born from lust and manipulation it seemingly evolved into something more than that but lust is still a high element in the current relationship.
But my favorite ending is the red dress scene and all the endings especially the pregnancy ending are basically called bittersweet ending which align with most NTR mangas and visual novel/game.
But the pregnancy ending is open to many interpretations but this is my interpretation.