The fact that Tirian heard nothing about a witch, yet the plot made sense to him, makes me think the witch plays a very small role in the film.

These days, they can put all of Leonardo Da Vinci's works on the head of a pin... I would be happy with a witch in VDT having a role about that size, or smaller would be preferable.

CORRECTION:
I just wanted to correct my own post regarding the possible role of a witch in VDT, as I found this on the news page of NarniaWeb (it would help if I hadn't left NarniaWeb for such a long time)...

UPDATE: There was a good deal of confusion due to the line about a witch being in the film. And while both Tirian and Specter from NarniaFans denied it, it was thought that perhaps they weren’t present when the information was given.

However, ChristianityToday released this statement today which puts any confusion to rest. “CT has since learned from Walden that the ‘new witch’ is not in the final version of the film, but was scrapped during script discussions.” So there you go, no witch!

So the grand epic journey of exploration has been transformed into a quest to find some super special magic swords to free Narnia from an evil curse? Is it just me or does this sound like the plot of a Power Rangers TV show?

that's just downright spooky.....it does sound a bit like it!