so what just happened lmao tldr pls

neiru was an ai like frill?
yup

where is neiru now?
dead

where is frill?
in the system. For better references, watch lain

why did himari commit suicide?
equivalent exchange; see "how tf is parallel interfering with the real world?" below

who are dash and dot?
AIs

how tf is kotobuki alive?
parallelism, one of the infinite side universes spit her back to the home world

how tf is parallel interfering with the real world?
call this world A; girl 1 dies at A. Girl 2 goes over parallelism to revive girl 1 which, as stated, is dead. What does parallelism do? Kidnaps girl 1 from an alternate reality and shoves her in World A. However, girl 1 cannot exist in two realities at the same time, so they are induced to commit suicide in their origin world and somehow get a free brainwash to avoid breaking down into an existential crisis. Plot twist, the frill entity is the core of all the revival system

how does this revived kotobuki know what her other self did in this world?
notebooks, records, we are not in stone age dude

why did koito commit suicide?
slipped from the rooftop while trying to destroy teacher's career with slutiness, yes, this is said exactly like that in the "special" episode

why did the revived girls forget their respective friend?
see "how tf is parallel interfering with the real world?" above

was sawaki sensei bad person?
no

and whole lot of shit unexplained
there's nothing left to explain; it's simply another series about parallel universes.

Think like this: what if they put a 6-year old to write a convoluted plot aimed at 18+ audience? It would be so pathetic no one would believe anything about that. But you cannot blame the kid; he's 6, no idea about anything on how the world works.

Putting writers to make a plot which includes parallel universes is basically that; string theory (which could explain these parallel realities) has been in an embrionary stage for aeons; no one can make that thing credible because it requires 10 dimensions to be explained. So of course it would be a swiss cheese series.

The only thing worse than spatial fantasy is time fantasy, where things can go back in time. Unlike string theory, there isn't any type of theory about the concept of time. Going back in time breaks logic at so many levels it becomes ridiculous when you try to make sense of them. Do you know this very acclaimed game called chrono trigger? It has every single example of time paradoxes which they leave conveniently unanswered because the true answer was probably too dark for kids: no matter what crono & party do, they would never save their home world, only an alternate reality where crono is the center of everything and nothing else is real. In the original world, crono died the moment he entered the teleport machine in the festival.

A simple example: My pal owes me money, but it'll never pay me back, so he gives me a knife instead.
Then I enter my time machine and go back 150 years in the past.
I then give my knife to his great-grandfather and tell him to pass the knife over the generations.
Then, at present time, I get my knife back.

After this very simple loop, tell me: where is the knife's point of origin? Who made the knife if it now is in a time loop? How did it enter that loop?

I can talk for hours about all flaws of time traveling, but this is beyond the point. Series is parallelism, which is also bad, albeit not at this extreme, but bad nonetheless.

There'll never be a sequel because none of the writers even know wtf is going on anymore.