who are the nazgul?

[Zigûr]

Originally Posted by mhagain

(aside from a note cited by Hammond and Scull that he was "probably" one).

The "probably" is important, isn't it? The point is that we don't know. In my opinion, thematically speaking, it's important that we know as little about the Nazgûl as possible. I think their anonymity is meant to enhance their horror.
I don't think it's unreasonable for the Lord of the Nazgûl to have been a Númenórean, but yes it's important to remember that he wasn't necessarily one.

Originally Posted by mhagain

There also seems no reason for him to have any particular grudge against Númenór aside from just carrying out his masters will.

I agree. In fact taking a Ring actually seems to be quite compatible with the fallen Númenórean mindset.

Originally Posted by mhagain

According to the Tale of Years, the Shadow fell on Numenor about SA1800, with the Ringwraiths first appearing about SA2251.

I suppose it depends how long it took for them to become wraiths, because 2251 is only 30 years after the reign of Tar-Atanamir when the tribute was exacted and 222 years after his reign began. If they received their rings during the tribute period, ie during the reign of Tar-Atanamir at some point, that's not that much time for a Númenórean to exceed his natural lifespan. But I suppose it could be enough depending on how old they were and how quickly the Rings worked.
The Númenóreans had been settling Middle-earth during the reign of Tar-Minastir, Tar-Ciryatan's grandfather, however, which was still about a century after the War of the Elves and Sauron, so there were plenty of opportunities when Sauron might have been dispensing Rings.

[mhagain]

Originally Posted by Zigûr

I suppose it depends how long it took for them to become wraiths, because 2251 is only 30 years after the reign of Tar-Atanamir when the tribute was exacted and 222 years after his reign began. If they received their rings during the tribute period, ie during the reign of Tar-Atanamir at some point, that's not that much time for a Númenórean to exceed his natural lifespan. But I suppose it could be enough depending on how old they were and how quickly the Rings worked.
The Númenóreans had been settling Middle-earth during the reign of Tar-Minastir, Tar-Ciryatan's grandfather, however, which was still about a century after the War of the Elves and Sauron, so there were plenty of opportunities when Sauron might have been dispensing Rings.

This is an interesting question, but I don't think you can set a fixed time from receipt of a Ring to becoming a Ringwraith.

Gandalf's words to Frodo in Shadow of the Past are the best indication I can find:

A mortal, Frodo, who keeps one of the Great Rings, does not die, but he does not grow or obtain more life, he merely continues, until at last every minute is a weariness. And if he often uses the Ring to make himself invisible, he fades: he becomes in the end invisible permanently, and
walks in the twilight under the eye of the dark power that rules the Rings. Yes, sooner or later – later, if he is strong or well-meaning to begin with, but neither strength nor good purpose will last – sooner or later the dark power will devour him.

From this it seems to be the case that it's not exceeding your lifespan that makes you a wraith, it's using the Ring often. Also, if you're well-meaning or if you're particularly strong, the process takes longer.

Smeagol/Gollum had the Ring for about 600 years, but "it was long since he had worn it much: in the black darkness it was seldom needed" so he didn't become a wraith. As Hobbit-kind he was more resilient too, of course. Bilbo had it for 60, and was already starting to feel "all thin, sort of stretched, if you know what I mean: like butter that has been scraped over too much bread". Also a Hobbit, also more resilient.

I think taking a median-point in Atanamir's reign and assuming that they used the Rings a lot, 100 years would be plenty of time for an already corrupted Man to become a Ringwraith. A total guess of course but it seems reasonable.