I think the biggest problem with the ML is that he is comically incompetent. Some traits might be exaggerated for narrative purposes but his is just out of this world. I can't see someone like him improving in an acceptable time frame unless manga magic (and since there are like 2 chapters left, it will be manga magic). FMC is equally fucked in the head due to childhood and while mom is WAY out of line with all the gaslighting shit, she is really not that wrong considering everything we've seen about them so far. So i don't know how a dramatic reunion suppose to fix any of their individual and pretty fundamental problems in 2 chapters.
They don't have to fix anything. Opinions on either character aside, you don't have to fix yourself before being in a relationship. You just get in a relationship and see where it goes. If you find someone that accepts you are including your flaws, who you accept including their flaws well that's great. Better if you both start to improve each other (not one sidedly (and Shinba was gaining self confidence, but we also saw Hizumi start to trust herself & others more; she hung out with friends again (her old friend/Shinba's boss made clear that she hadn't in a long time) and was making new ones)). But none of that is even a prerequisite. All either person in a relationship has to do is be happy. Not forever just in the moment. If that stops or things break down, okay. Relationships end all the time. Happily married couples get divorced after 10, 20, 30+ years of being happy.
I agree that it'd be impossible and just downright a bad ending if Shin a and Hizumi magically became different people, not better people, different people. Because their character flaws aren't things that can be fixed and go away completely without them swapping bodies. That's going to be a lifelong journey for both of them with both having started in childhood (it takes a LOT of work to change mindsets and feelings we develop through childhood especially because our brains are just trying to avoid pain we felt during development). But they really don't need to. They aren't breaking up because of their problem or any character flaw causing their relationship to fall apart (they're kinda perfect for each other in the same way they're terrible for other people; Hizumi really lucked out because she just wanted a actual scum of the Earth moocher and her previous dating experience was the type of men that'd cheat on her). They're breaking up because someone else told them they'd be making the other person happy by doing so, played to each person's insecurities while doing so, and then created (manipulated) a situation where they'd break up without either person even actually saying they want to or should.
In writing you don't want to resolve a character's flaws, but to leave a reader knowing that they've been confronted and that we can trust the characters to continue to work on them past whatever point we end on. We have that for Shinba actually. He's pretty much good. If this Manga ended right now we'd leave knowing he's going to keep trying his best to improve himself, doing so at whatever pace he can, and all while making Hizumi happy and feeling like she's his number 1. Money isn't an issue (if she had it her way Hizumi would handle all of it) and Hizumi doesn't want a partner that's completely independent. Shinba also doesn't even need to buy a ring or be the one to propose. That can just as easily be the woman in a relationship. The one who still needs to confront her flaws and take a more active role in improving herself is actually Hizumi. She's entirely dependent on feeling like Shinba is dependent on her, and she struggles too much anytime we see him even try to improve himself. She's gotten better, but we need to see more active and conscious effort.
The mother is an opportunity for the author to showcase that Hizumi is ready for that and there's a future for her. When the MC's childhood friend entered the fray he very strongly refused to leave Hizumi for her, and when his sister came in he strongly pushed for living away from her with Hizumi. And while Hizumi had to help the sister become independent, it was still Shinba that made it clear he didn't want to stop moving forward with his life or relationship for his sister. Those were people from the MC's life that came in trying to drag him back to be the person he was, so that he'd stay in their lives & not in Hizumi's (who they thought would take him away). This is the first time someone from Hizumi's past is doing the same and she needs to properly stand up to her mother. When that happens they can move forward with their relationship and we can even be kinda confident in it, but more importantly she can move forward with her life. She doesn't have to stay with Shinba and Shinba doesn't have to stay with Hizumi. There may come a point when they aren't happy together or their flaws don't mesh anymore. Who knows. A good ending is just feeling confident the main characters will keep moving forward.