I think the perfect ending would be to never explicitly tell readers what the truth is, and to only present how either narrative could be true.
Ultimately the only reality of a work of fiction, regardless of its in-universe truth, are the feelings and reactions it elicits in its readers-- everything that transpired up until now is not for nothing.
When I started reading this manga, I didn't believe Seiichi would be able to pull himself out of that hell, ever. Following an abused child as he internalizes his mother's ultimate betrayal and gross behavior, parses and processes his extremely complicated feelings toward her and himself, realizes that a parent does not inherently deserve to be in their child's life, develops a sense of self worth, gradually comes to claim control of his life... Even if it ends up being a lie, that transformation was truly handled masterfully, especially reading it as it released over months; growth is slow and rarely steady.
When I started reading this chapter, I didn't believe Seiko could be redeemed, ever. While I would've loved a hopeful ending for Seiichi, I looooooove to have my expectations subverted-- what if this is a tragedy for Seiko too??
The idea that her unsettling smile was forced out of fear, not a sociopath attempting to mimic the rest of us; if this whole time she's been trying to protect other children, the girlfriend, Seiichi from himself... Of course it's not okay for an adult to abuse a child, even if that child is killing animals and displaying psychopathic tendencies, but what if she never pushed him, he just fell? Dad blamed her and she blamed herself, so she accepted the narrative? And maybe she only recognized Seiichi's nature so early because he got it from her?
I'm here for whatever, thank you Oshimi.