Here is some of my theories and thoughts. All of them have little to no evidence to back them up, but I like them, so here they are.
Ami = Sekai isn't entirely far-fetch; there has been discussions regarding if the current Ami is the
real Ami or not after seeing the last scene of Lamb Legs (there we saw a seemingly sweet-natured and not sex-depraved Ami). Perhaps the yandere girl we know and love is a teenager Sekai that has already swapped with her daughter, whatever that means.
But still, I'm more inclined toward believing Ami and Sekai are separate people.
1) Sekai has this omniscient and witty aura whenever mentioned, but Ami is just an adorable dumbhead (Ami does has enough talent to write poets though!)
2) Despite many of Sekai's appearance being explainable as Sensei's trauma-induced hallucination, I think a free-floating spirit Sekai indeed exists. Implications are Yasu saying
You are surrounded by whispers/Someone misses you very much, and her involvement in Futaba/Makoto lust scene. So a Sekai spirit and an Ami who is not Ami but actually Sekai existing in the same world and time seems unnecessarily complicated.
I can totally see Sekai wanting to "continue with Akira" through Ami, but more like through assuming control of Ami
As for if Akira ever loved Sekai or not, I think he definitely did (why does everything hurt so much then?). In Mother's Milk, the slideshow is concluded as
For both of you, it was love. A distorted and forbidden one, but love nonetheless.
If Akira really wanted to get rid off her by orchestrating a car crash, then he shouldn't feel
that traumatized by it. He would basically remove a clingy molester and a brother who he felt inferior to and was probably the cause of Saki's favoritism. He might feel guilty if he caused it, but so far Akira seems to really miss her but is also trying his best to tell himself
she is already gone to combat the fact that he could occasionally
see her.
If that accident really causes this level of turmoil for him, Akira must has loved someone who died that day very very much. Since we are not seeing horny balls of light in Akira's hallucination, I am gonna go out on a limb that the person is Sekai.
