My god! These theories are very good! And i think with the amount of detail you've put in i wouldn't be suprised if it was almost true! i think it is very likely that snape was in love with lily, that would explain his hatred for james!

I think we've already established that the locket horcrux is at grimauld place because in the 5th book when they're cleaning out the house harry, ron, hermione and ginny come across it but leave it because they can't open it, whether they chucked it out we don't know we also don't know whether it was something mungdungus took which would explain the importance of jkr including harry's meeting with him in hogsmeade!

I also think that snape is still definately a double agent and his loyalty still resides with the order, he and dumbledore planned his death, hence the reason dd pleaded with him at the end of book 6, he will use his new found status to break down voldemort and the death eaters from the inside and continue to pass information to the order. This also explains why dd always trusted snape they've obviously had this plan a long while and i don't think that such a wise man as dd would make the mistake of trusting someone who could so plainly be a de!

I think neville will also have a lot to do with the ending of the book, considering it was a 50/50 chance that it could have been him instead of harry, and he's been a semi-main character throughout the whole series! And a devoted da member! He will get revenge on bellatrix lestrange for the torture and murder of his parents.

Now the dursleys....petunia has always shown her dislike of her sister but i think that's a bit of an act...she simply resents the fact that she wasn't special...perhaps she's a squib? And being a squib made her give up on magic completely and turn to muggle life, dudley however has also got some magical ability which has been supressed desperately by his parents, much to dumbledore disappointment that he couldn't come to hogwarts.

Petunia knows alot more than she lets on and will eventually reveal all to harry when she realises the danger everyone (including muggles) is in! She will help reveal more about harry's parents pasts and ultimately give harry some of the information he needs to defeat voldemort.

Harry may well be a horcrux himself or maybe just his scar but if he himself is a horcrux then in order to completely get rid of voldemort harry must die which i think would be a very sentimental and beautiful ending to a very heroic life. However if it's just his scar that's the horcrux then he need not die

But of course this is a very distant theory as i'm sure if dumbledore had any idea that harry might be a horcrux then he would have warned him before hand! So i'm still quite sceptical lol!!