Certainly the story would be more interesting as this has never happened before...buuuuuut...
I know you said that you do not believe that he is one of the seven, but an accidental extra, so that being the case, how would he ever know that he was a horcrux. I mean, say he weakens LV, has destroyed all 3 horcruxes he searches for, and then he avada kedavra's LV. Now what would happen if he's a horcrux? I am not asking anyone to speculate, that is not what this thread is about, but it would not fit w/ continuity for anyone to know that LV was
not dead. Even if he had one last horcrux in Harry, his spirit would have been ripped from his body, and he would appear dead. I can't imagine that everything that must be done to lead up to this battle can fit in one book, along with the re-resurrection of LV.
I suppose the last sentence could be, "and Harry stared down at LV's lifeless body and smiled, yet what he did not know was that an unknown horcrux lay hidden in him; in his scar." etc...
As I said before, I just don't see Harry being a horcrux, but hey...I've been wrond a couple times before
, so who knows?
Another thing though, as much as I respect JKR as a writer, I just don't believe for a second that she had everything tied up so neatly when she planned these books out from book one. I think she had an idea where it would end, but the general plot thickness came as she went. I'd say by GOF she had the last 3 mapped out, but before then, she was building on what she wrote already. This is just my belief, but when you look at the style of the first few books, and the level of continuity, they are just not equal to the most recent 2 books. this is acceptable because she was writing about a boy being introduced to a new world, so she could introduce everything as she went. This is why I don't think horcruxes will play any part after Harry destroys the hidden ones. Everything will lead up to the classic battle she has planned since before she even invented horcruxes.
Whew, what a huge post, so sorry!