Maybe...Susan and Peter come in when Lucy is looking at the beautiful spell....or some random spells....?????

Disney wasn't responsible for Susaspian, Andrew Adamson made that decision all by himself. And what are you talking about Disney putting their people in PC? PC was directed by the same director, produced by the same company, and the screenplay was written by the same writers.

I just had a theory:
what if the whole scene with the four Pevensies was a dream that Lucy had before they get to Narnia?

^That is definitely possible, but where are they in her dream, and why couldn't it be Edmund's?

^^ Well Lucy is the one who has "dreams" of this nature and I would say she is connected the most with Narnia (although I would say Edmund would be second.) It could be anyones dream though (if that is what it is.) It just seems to be in character for her to have dreams like that. I like the idea.

Didn't the screening of the clips mention that Lucy saw herself with Susan in the mirror in the page of the Magician's book, and she could see that Emund and Peter were there too. Isn't that sort of what the shot of the four of them walking together could be?!?

^^ yes, and it makes no sense to me. Is this just an excuse to push Peter and Susan into the film??

I don't know exactly what the shot is but from the reports and latest trailers and posters and interviews, its clear that Peter and Susan won't return to Narnia.

So for right now, their appearance in the film is the least of my worries.

Yeah, it is an attempt to shoehorn Peter and Susan into the movie so they can slap them on the marketing and trick non book fans into thinking they come back to Narnia.