(Um, I apologize for totally failing to reply for days. School has just started up again, and it is being like hit by a truck, or something. So I hope I answer things, because this is such an interesting discussion.)
Re: author intent and canon and such. I generally hold canon about like MRW does--with the core books first, and then auxiliary books, and then interviews (most recent to least recent). Notes written during the writing process or anything edited and published by someone else after the author's death I don't count as canon at all (like JKR's class list--interesting and cool, but not canon). And anything subjective ("I always considered Character X to be a nice person") isn't canon either, and of course no one who is not the author can establish canon (this from some traumatic experiences with later Oz books when I was little). And I wouldn't consider speculations as to what an author might have intended to be really relevant, except maybe as a bit of extra evidence somewhere. [And I do certainly love hearing extra bits about the characters from JKR, but they occupy a sort of weird space in canon in my head--I consider them canon but I don't think a writer needs to be compliant with interview canon to be canon-compliant, so to speak. And she's got every right to tell us, especially as she's generally asked by fans.]
The problem with proposing that the Green and White Witches were the same purpose, and basing your proposal on the fact that there is nothing in the text which contradicts it, is: there is nothing in the text which contradicts a lot of things! You could propose that Susan had a dozen engagements that she broke when she lived as a young adult in Narnia -- there's nothing in the text that contradicts this, but then why would there be? It's not an issue.
If you want to posit that the Green and White Witches are the same person, then you have to find something in the text which supports this. You can't just say it is so based on the idea that nothing in the text refutes it. Where is the support for this idea?
Actually, all I require for fan theories is that they be compliant with canon (I guess it might be helpful at this point to say that I usually look at this sort of thing with fic-writing in mind); if someone can sell me on the idea that it fits in with canon, I am all for it! In fact, there's
a VotDT AU where Lucy stays with Caspian that's been started on ff.net; the opening scene is, I think, meant to be the point of divergence but I actually think it falls very well in with canon. There's no evidence at all that it happened, but it doesn't change the story to think it did. [This is sort of complicated, because there are always disagreements over what fits in with canon and what doesn't--is it easier to believe that Lucy never had a maid/lady-in-waiting/some female companion on VotDT because we never see one and *surely* we would have, or is it easier to believe she must have, because in a country that concerned with chivalry surely she would have, and surely someone would have told Caspian it was a bad plan to let her sleep unchaperoned in his cabin?]
With that having been said, I wouldn't bother arguing about the witches if I didn't think one witch was a valid interpretation (I don't think it's the only interpretation; I like them both and I go back and forth on which one I like at a particular time. I don't have any sort of personal canon here). I think that the combination of Jadis's immortality, the hag offering to bring back the White Witch and being taken seriously, the similarities in their methods (which I see, but I am totally okay with being the only one here) and sort of...maturation of their powers, and the similarities in their motivations to look at them as one person.
I also think it's entirely possible that Lewis would go, "What? Of course they aren't the same!" or that he would go, "Well, yes, they are," or that he would say, "Pft, I've no idea. I wanted to leave it open to confuse you/to give myself more options for TLB/just because." I certainly don't see why he would necessarily have to tell us outright if he wanted them to be the same; plenty of authors throw in weird little things that they don't bother to explain or don't have the space to explain or whatever. (And I don't think it's particularly relevant anyway; I see enough evidence in the books themselves to support it.)