Sei has already started anew once so I think he'll start again. As for his mother, her life was also rotten from the start. It was her fault but also it wasn't. It was her parents fault but we don't know about them much so may be it wasn't. I think the life is to hard to blame anyone. The older you become the better you understand it. Any person should think about himself and judge only himself. He should try to overcome his bad background, as much as he can, so I hope Sei manage to do it. He already did his first steps, talking to his mother.
Regardless of whose fault it was, it's undeniable she impacted him in a manner she'll never be able to take back, while he was under her care nonetheless, even if she wanted (which she doesn't). I do agree blaming anyone won't solve the problem, so this is about recognizing responsibility for something she chose to do despite knowing the consequences. And honestly after seeing the absolute disregard she had for him, and still having to care for her, while he has no support himself to help digest everything that is going on, it's no surprise interacting with her in any way did him more harm than good. Maybe in the future, when he's in a better place, he'll be able to look back with maturity and understanding, but that's not the him of now. Btw sorry if this sounds like a rant, I still remember the little boy he was and everything is so unfair
I am agree with you that what Seiko did were wrong and it makes her son's life hard even now. Your point of view is correct, but it also means that your environment is healthy. It is mother's responsibility to give her child all the best. But what if she did it? The hardest part of this story is that Seiko really did her best. It's hard to accept but not to accept it means a blind alley for Sei. The older you become the harder to judge human's behaviour. Especially if you have already met something too layered to give an assessment. For example, postpartum depression, which wasn't is this story, but exists in reality. How about it mixed with undeveloped society? The result may be disastrous.
As for Sei's future. Judging by what Sei saw in his "dreams" about his mother and Shigeru, Sei is much stronger than Seiko. He managed to understand his mother and her past so he can do it for himself too. I see him as a strong and empathetic person. I hope that he will be fine. Cause what were happening to him was his own life and his parents were his own parents. He couldn't have any other. If he understands what it means and choose his own path he will be okay.
His mother ruined his life to such a degree that I, honestly, can't even imagine what a future would look like for him. He needs help but is unlikely to get it, has just lost his job, and has a rotting corpse in his living room... I would say things can't get worse but they probably will ╥﹏╥