I'm not sure if its worth making the Last Battle into a movie at all, but it definitely would not do as a two-parter. "Part 1" as you call it, could possibly work as a self-contained apocolyptic war movie. You have a point there.

The problem is making Part 2 into its own movie. The Real Narnia part of Last Battle is more of a metaphysical poem, than a story. It has no character development, no conflict, the only goal is to go further up and further in. You can't make a movie based on that. Maybe the screen writers will cook up some tacked-on conflict, like the people have to go further up and further in to escape a tsunami. You'll think I'm joking until you remember the scenario of people being sacrificed to an evil mist.

I think the best way is to end with the characters escaping through the door. As you yourself pointed out, the shift from the Narnia Apocalypse to the Real Narnia is jarring, and anti-climax.

Oh, I would hate it if they left the ending out with them going to the New Narnia. Plus they kind of need it since the rest of the book is so depressing. And we need to establish what happens since clearly Narnia isn't restored. Otherwise people who haven't read the books are just going to be thinking "What a stupid way to end it -- everybody dies!"

But they don't even find out they're dead until after they get to New Narnia. If the movie ends with Jill or Eustace peeping through the ajar door before closing it on darkness and void, the non-reading audience won't know they're dead either.

I do think they'd need to cut the ending down a bit, Anhun - but probably not that much. People need to see how joyful the ending is. Perhaps end it with the group setting off further up, after Aslan explains where they are and bounds off? The Friends are all running and bounding and laughing and the camera pulls back to the vastness of the new Narnia and Aslan's Country?

Perhaps some of the things from the end can be woven into the story before they enter the Stable - for instance the talk with Emeth. I can imagine them capturing him as the guard on Puzzle, then they talk to him (slightly different since he either hasn't yet met Aslan or it would've been under different circumstances) then maybe he escapes or is freed by the other Calormenes but later enters the stable similarly to the book scene. Then at the end they just see him again, perhaps talking with some of the other characters from earlier on.