[Man-of-the-Wold]

I've always thought that either explanation was possible. He could have left the Ring at Barad-dur, where the Nazgul could be trusted with such security, quite effectively. He sent them to hunt down the Ring in the FoTR, because he trusted them in just this way.

Also, he could have had it in Numenor, kept it concealed (just as any ringbearer not wishing it to be seen, as with Gandalf and Narya all those years), and then brought it back, even as a fleshless spirit escaping from the downfall, just as Glamdring and Narya were not lost to Gandalf after the Battle of the Peak, although Gandalf sort of had a body, but it seems that Sauron's sprit could probably cosset the ring well enough. Heck, he had no physical form by the time of the War of the Rings, and yet he would have still worn it somehow.

What troubles me with this question: Is did he really use it to seduce the Numenoreans? ... or did he actively take it up again for the first after an apparent interlude, as implied in "Of the Rings of Power and the Third Age" regardless of where it had been during that interlude.

Again, if the Numenoreans are simply manipulated by the Ring, that makes their culpability much much less, I feel; the Ring is incredibly powerful. This would also belittle the state of corruption to which the King's Men had been sinking for generations. Also, Sauron's actively using the Ring would have likely been obvious in a very imposing way, without the subtly that he is described to have employed.

I think the Books, both the Lord of the Rings Appendix I(b) and the Akallabeth point to Numenoreans who were seduced by their own greed, infidelity and Fear, which made them rather ready victims for Sauron's evil lies. Melkor did not need to directly exert raw power in twisting the minds of the Noldor, only cunning, and the same was true for Sauron and the Elves of Eregion. It would seem that little more should have been absolutely needed to take the Numenoreans to the final stage of their downfall.

Had JRRT seriously meant the Ring to be the cause of the Numenoreans' Fall, it would have also been Numenor's, and not only Isildur's, Bane, and somehow this would have been indicated in the Lord of the Rings' Appendix, or later works to revise the Akallabeth, post-LoTR. To have done so, however, would have undermined JRRT's philosophical emphasis on the corruptibility of all peoples, especially prideful and fearful Men. Also, when Sauron was actively wielding the Ring in Middle-Earth during the Second Age this would have already affected Numenor through the Black Numenoreans that were connnected with the King's Men or their earlier forerunners. Three of the Nazgul were such Numenoreans, even if most Numenoreans, including Ar-Pharazon were unaware of the Rings of Power, per se.

Nevertheless, Numenorean society became very wicked in a way that the Noldor never did, and this may reflect Sauron's personal will at work over less devine people, but such horrid, debased behavior is not at all alien to human history in Middle-Earth or in this world. And nowhere was it said that the Numenoreans were necessarily morally superior to other Men, simply more enlightened, valiant, longer-lived, etc.

As for the Letters, they are great for some insight, but they are not works that JRRT ever envisioned to see published, and they are in fact his personal replies to some individual or another. They are remarkably consistent, but they also may reveal only passing notions that he had and used to satisfy someone and not necessarily thoroughly thought-out conclusions in line with even other aspects of his then evolving legendarium.

In summary, it is best concluded that Sauron most likely had possession of the Ring in Numenor, and was able to rescue it with his spirit from the deluge. It also should then have contributed to his cunning and the power of his Will with Ar-Pharazon and others, and to the evil influence spreading among the Numenorean people. Nevertheless, he did not overtly wield it, in fooling and seducing the Numenoreans, and the Numenoreans were not really corrupted by the Ring in any direct way. Rather, it was through their own failings, coupled with Sauron's subtle mind, that they fell into the trap that caused their downfall.