Thanks for the spoiler ... I haven't read Narnia, but am looking forward to seeing it.
But, if one were to claim that any movie in which a good character dies (or is represented as being as-good-as-dead, and is then resurected; then can't we also say that E.T., Superman, 5th Element, Highlander,Finding Nemo,Little Mermaid, Sleeping Beauty, Matrix, Snow White, and Peter Pan are all Christian movies too? After all, Tinkerbell is for all intensive purposes dead; then she comes back, as did Neo, Nemo, Superman, the 'Supreme Being', and the rest.
This is something that as a movie producer, I wouldn't focus on. I'm not involved in the movies, lacking both talent and desire. But consider; to proclaim a movie 'Pro-Christian' is to invite attacks on multiple fronts. First we have the 'Christians' who say that YOUR intrepretation of Christ isn't the same as THEIRS; so obviously YOU must be wrong and evil. Then we have the Jewish, Muslim, Buddist and Wiccan people who WERE looking for an entertaining movie; but don't want to see a movie counter to their religious beliefs.
And thirdly, I question the timing of this discovery. What an amazing coincidence, a letter lost to all for 30+ years is discovered just HOURS before the US theatrical release. Some may call it coincidence, I call it highly suspicious.
You make a good point about other movies with what could be considered Christian themes or elements, but that really aren't intended as Christian movies or books. But with Narnia, it seems obvious that the author's intent was to fashion a Christian story via a thinly-veiled allegory with talking animals. I just find it interesting that Lewis' stepson would deny what Lewis himself said of the story.
As to the business considerations; yes, I think one of the reasons Gresham responded as he did is because he doesn't want the film to be 'tainted' in the press as a Christian proselytizing vehicle for unaware audiences. Yet at the same time Disney has been reaching out specifically to the Christian community with its marketing.
I don't see why the stepson couldn't just say something along the lines of 'yes, Lewis wrote this as a Christian allegory, and we have been faithful to that spirit in the film adaptation, but at the same time it is not an evangelizing tool and is not so overt that a non-believer would be uncomfortable, and as such we have made it so that it can be enjoyed on any number of levels, from a religious allegory to nothing but a fun fantasy/adventure film.
That seems to me to be a better, offend-noone answer.