I don't think two years of pre-school before first grade is unheard of even in the US. With, pre-k and kindergarten you can absolutely have classmates for 10 years by 8th grade. And I guess it might be different in different places, I honestly thought Japan had 3 year olds going to some...
What kind of shitty story turns into a harem 19 volumes in. That wouldn't make any sense. This arc will at most be another one of those second female leads last hurrah before giving up and anything else would be stupid.
This guy totally misinterpreted the "Memories" chapter and thought it implied that Nishikata wasn't Chi's father. I'm pretty sure there's nothing else to imply anything like that, but that interpretation of that chapter is totally wrong.