[Gwaihir the Windlord]

Then how do you explain the Dead Men of Dunharrow, Gwaihir?

As has been before mentioned, Illuvitar (and the Valar) obviously considered Isildur enough of a good guy to let his curse take effect; and of course the Men were serving Sauron, making them not just cowards but evil cowards, too (the very worst kind of evil bastard). Porobably the voice of Illuvitar spoke to Isildur inside his head, perhaps subconciously, telling him that he had the power to curse the Dead Men so.

As for Beren, Inzil, I'd have expected you to know better. He wedded an Elf. There was a fate set apart for such Men. And since Beren had to pass through Mandos on his way to heaven, he obviously resisted as much as he could and tarried there a while, so as to be with Luthien. The Valar took pity on him and sent him back. They had the power to do that, I believe, with Illuvitar's authority.

.....gave back in terror, believing it was his unquiet spirit, and the women wailed and covered their eyes.

Unquiet spirits... we believe in them, don't we? ME unquiet spirits, those of Men, mean, probably could resisit for a while. Not for long though. not more than a few days.