With or without the book, you’re dead right.
I wonder how much they’ll make of the guerilla war by Old Narnia vs the Telmarines? It’s not made much of in the book, but it would certainly help to fill out the film (in the BBC series, they got PC over in two episode and tacked VoDT straight on to the end).
“Draw the circle. Prepare the blue fire…” 
I hope they flesh out the Old Narnia/Telmarine fighting. It would give a good picture of how desparate the situation was by the time they blew the Horn. And I hope they leave in the Narnians winning because of the reawakened forest.
Looks like they might. In the trailer there’s a shot of Lucy with those animated leaves they used to signify living trees in the last movie.
Squee! Can’t wait for this one to come out. I agree that Caspian looks too old, albeit hot, but as long as he doesn’t look older than Peter I guess it’ll be all right.