I think she has immortality regardless of the prophecy of Cair Paravel. My interpretation of the apple's function is that it tied Jadis's life to the life of that world, which is why she didn't completely die even when Aslan killed her in LWW -- and was therefore available, in some sense, to be potentially summoned back into physical being during Prince Caspian. I'm not even sure if she dies completely at the end of The Last Battle, because so far as I can tell, the Narnian world does still exist beyond the stable door; it's just a dark, frozen, motionless wasteland for all eternity... and Jadis is trapped there, bodiless, but perhaps still aware on some level.

Which, I grant you, is not a fate she would like, though it does satisfy the letter of her desire for immortality. :-)