Right this concept is being based off the idea of Voldemort either having a mental link or having part of his soul stuck in Harry.

As we know, to all appearances Dementors pull out and force a person to experience a person's worst memories, however do to either the mental link or the horcrux Harry instead experiences Voldemorts memories whether at the beginning of book 3, with the single Dementor, the quidditch pitch with several or the end with a whole horde of Dementors.Que mental chaos and other changes depending where a person wants to lead this.

Also, spawning from this though I dislike the of putting Dumbledore on the horribly manipulative side of the spectrum, Harry has been obliviated of his memories when he was younger however, due to memory charms just suppressing instead of obliterating the memories he gains them back.
-Thus we get the manipulative evil/goody Dumbledore.
-Though this could also lead to several other people obliterating him, might be a good start to for a wandless Harry, where he's done it as a child however the Ministry obliviates him to protect the wizarding world from discovery due to 'accidental magic'.
-Edit* and yea I know its horribly cliche but I'm sure at least this twist might be slightly original...the rest of it however....


Finally I'd like an opinion on why Neville didn't pass out in the books. Was he just not there when Alice and Ted were being tortured? Because no matter how I look at it memories of agonizing screams tops flash of green death.

 

Wasn't it mentioned somewhere that he was hidden upstairs? Or was that a fanfic?

Your first idea is good. It could make a novel approach for a kind of memory cascade into Harry's head, allowing him to know Voldemort better than anyone else.

I don't really like the second idea though, its just too based in cliche.

 

So what you're saying is that Harry experiences Voldemort's memories instead of hearing his mother dying when he comes into contact with a Dementor... Could work. Seems original at least. Third year could be the building up of pressure for the memories to totally take over (if Dark Harry is the way you want to go) or for his personality to merge with Harry's. End of the year with the hundred Dementors would have to be the breaking point, though whether you do it before he's gone back in time or after would have to be figured in.

This part I'm confused about - why would he need to be obliviated if he hasn't been in contact with the Dementors so he doesn't experience the memories. The first part of this was quite good and very original, the second just repeats every other indy!Harry fic I've ever read. The only way it could be more cliché is if you had the Weasleys stealing from him.

First things first - Neville's father is called Frank. Ted Tonks is... Tonks' dad. This way you could easily use the memory charm route, though its just as cliché as the previous obliviation route.

Second... Neville may just be more at peace with it than Harry - he's been able to visit his parents when ever he wanted, though I doubt that's much of a comfort. I dunno really, maybe he wasn't in the room when they were being tortured, with silencing charms over the room... Possibly.

Aekiel