[Inziladun]

Gordis

They were supposed to go there, they were summoned, but they could refuse the summons and stay as disembodied ghosts in ME. It was against the Design of Eru, so such Elves became easy prey to Morgoth and his followers.

I don't have the HoME books, nor have I read them, so I was drawing from the 'core' works - LOTR, UT, and The Silmarillion. I knew of no instance of an Elvish spirit remaining bodiless outside Mandos. Even those who died in Valinor were there. All the HoME information is very interesting, and I must make some effort to acquire those volumes.

The "Last prince of Cardolan" whose barrow it was (see App. A), died in 1409. The Wights entered the Barrow soon after the Plague, in 1636. So the corpses had only about 230 years to rot naturally. If the conditions were good, the air dry etc., in such a short time the bodies could have been quite nicely preserved, especially if embalmed properly. And I guess Dunedain (with their obsession with Death and tombs) did embalm their dead, much like Egyptians

The Barrow Downs didn't seem especially dry- green grass was everywhere, fog was in abundance, and while the hobbits were in Tom Bombadil's house they noted:

The upper wind settled in the West and deeper and wetter clouds rolled up to spill their laden rain on the bare heads of the Downs.

FOTR p146 (paperback)

It is also interesting to observe that in the Moria Chamber of Mazarbul, presumably a far drier environment, we have this description:

By both the doors they could now see that many bones were lying, and among them were broken swords and axe-heads, and cloven shields and helms.

FOTR p. 360 (paperback)

Those bodies, apparently the result of fighting between Balin's Dwarves and Orcs, had been there only thirty years or thereabouts, and already had decayed to bones.
Embalming could explain this, as the Númenoreans certainly practised the art, but I would question whether they did it as a matter of custom on all their people, or just their Kings (and later, Stewards).

There is some textual evidence. Firstly, we know that the Witch-King had visited the Barrow Wights right before Frodo et al. were trapped in the Barrow.

Quote:
The Black Captain […] himself visited the Barrow-downs. In notes on the movements of the Black Riders at that time it is said that the Black Captain stayed there for some days, and the Barrow-wights were roused, and all things of evil spirit, hostile to Elves and Men, were on the watch with malice in the Old Forest and on the Barrow-downs.-UT, "Hunt for the Ring"

I believe the Witch-King reminded the Wights that he was their master and explained to them the current goals. They had to know he wanted the One Ring. Strange it is that the Wight hadn't summoned the Witch-King straight away. Instead it started this strange dark ritual involving the three hobbits, but not Frodo. I can't understand the Wight's behaviour…

Indeed. If the Wight knew of the existence of the Ring, why wasn't it taken from Frodo immediately? That is why I simply cannot believe that the Wights did know. The Witch-king stayed there for a while, and his presence acted as a catalyst to increase the hostility of other evil creatues in the area. But did he actually tell those creatures what he was doing, and what they should look for? Again, the actions of the Wight that took the hobbits seem to indicate it was doing its own thing, evil in itself, but not necessarily in lock step with the goals of the Witch-king.