I think this update confirms the theory that Kaori's accident and Sekai's accident are the same.
We've known for a while that Kaori was hurt severely in the accident, so much so that she was in the hospital for a long time, to the point that Yumi had assumed she'd died.
So she likely had to receive an organ transplant to stay alive.
I think the giant spider tattoo is there to cover up the surgical scar. I think she wound up with Sekai's heart. This is why Sekai appeared to be able to take over in that next to last scene without looking all sex-zombie like in Toys.
I think this is reinforced by Rika talking about getting a tattoo to cover up a cult brand. Creating repetition of using tattoos to cover wounds of a past you'd prefer to forget.
In one of the old happy scenes, we hear the sounds associated with someone being on life support. In one of Akira's memories, he talks about pulling Sekai out of the wreckage. I think she lived long enough to wind up in the hospital on life support, but was in a condition where she'd never wake up. And as the next of kin, it had to be Akira that authorized the removal from life support so that her organs could be used to save the lives of others.
That's why he was such a basket case after the accident. Not because of her death by itself, but because HE had to be the one to make the call to make it official.