I have a problem with the entire sent back in time thing. firstly the idea of time travel hasn't even been touched on before.
Except it has. It probably got lost in all the psychobabble.
Even god can't send his prophets back to stop things happening.
Let's not go there.
And apart from her role as narrative bookmarker why would she choose to manifest there. It serves no purpose. I know she needs to watch Shinji, (why else does she need so many eyes) but why then.
He only saw her at that moment; she's probably always watching him.
furthermore the idea that she can travel back in time throws into doubt that any of the reis seen through the story are real.
No, it doesn't. At all.
What seems more likely is that Shinji is viewing the series in retrospect once he ahs been absorbed, which is why some characters seem to be manifestations of his own tendancies feelings. As he has been absorbed into Rei by then she can appear in his memories.
What? That's even worse than Rei time travelling. You're complaining about the time travel idea not being brought up earlier and casting doubt on Rei's reality and you counter with an idea that is even less supported and casts doubt on literally everything by turning everything into an imaginary retrospective of something we've never seen.