The Mewlips

[Encaitare]

So were the Mewlips the wights inhabiting the Dead Marsh?

I think it's pretty likely.

Could be.

The part about the Will o' the Wisps was particularly interesting; the book "Faeries" (Brian Froud and Alan Lee!) offers a blurb about them:

In some remote areas a curious light resembling a flame is sometimes seen flickering in the distance. Traditionally known as ignis fatuns [I think that's what it says, it's hand-written], this is also called Will o' the Wisp in the British Isles. Although there are a number of theories on the nature of this phenomenon, no entirely satisfactory explanation has been put forward.

Seeing as the good Professor was British, it may very well be that he had these in mind when he wrote about the Dead Marshes.

I think a little fanfiction attempting to explain these curious Mewlips is in order.