[Zigûr]

Originally Posted by Ivriniel

Adapting from the last two paragraphs--the Nine Rings--appeared able to extend mortal life. I am wondering what the Eldar's intention was for that, were Sauron not to have perverted the Rings. I seem to remember that Celebrimbor and co had a hand in the making of the Rings of Power beyond the Three.

...what actually was the Elves' (not Sauron's) intention for the mortal form, for The Nine--before the Elves twigged who Annatar was--then when Sauron was detected speaking the Ring Spell at the Sammath Naur at Day Zero of the One. I suspect the Gwaith-i-mirdain knew how to extend the life of Men, almost infinitely!

As far as I can determine, the Elves had no plan for the mortal form. The Great Rings (the Nine and the Seven) were not intended for use by mortals, at least from the point of view of the Elves.
We must remember that there were three different 'types' of Ring:
Elven-Rings (three, forged by Celebrimbor alone)
Ruling Ring (one, forged by Sauron alone)
Great Rings (sixteen, forged by Celebrimbor and Sauron together. There was no difference, functional or otherwise, between the Seven and the Nine; their delineation is arbitrary based on how they were allocated by Sauron. Their different effects were only due to being used some by Men and some by Dwarves)
I would argue that the Gwaith-i-Mírdain intended to use all of the Great Rings themselves in the beautification and maintenance of their "separate, independent paradise" (Letter 131): a well-natured but ultimately hubristic act, and one which was the second 'Fall' of the Elves. The weakness and effects were built into them by Sauron, I believe, independently of any intention on the part of the Elves.

(Poor guy, he was hot for Galadriel and made the Elessar to try to woo her, in one take on the history. Was it just that he was not that good looking for an Elf? He never married and had kids and no-one ever explains why an Elf doesn't do that).

To quote from "Laws and Customs among the Eldar in Morgoth's Ring:
"among the Eldar, even in Aman, the desire for marriage was not always fulfilled. Love was not always returned; and more than one might desire another for spouse. Concerning this, the only cause by which sorrow entered the bliss of Aman, the Valar were in doubt. Some held that it came from the marring of Arda, and from the Shadow under which the Eldar awoke; for thence only (they said) comes grief or disorder. Some held that it came of love itself, and of the freedom of each fëa, and was a mystery of the nature of the Children of Eru."

What was the actual nature of the peril the One had for the wearers of the Three. We know the Three could communicate telepathically.

I think the greatest danger was to what was achieved by the Three, not for their wearers specifically. As Gandalf told Frodo: "If he recovers it, then he will command them all again, wherever they be, even the Three, and all that has been wrought with them will be laid bare, and he will be stronger than ever."
Sauron would learn everything the Elves had discovered or devised in the Third Age. He would also be able to dismantle the defences the Rings provided for Lórien and Rivendell, making their destruction a simple matter. He might, perhaps, have been able to enslave the minds of the bearers, as he had done with the Nine, but it's unclear to me how that would affect the Three.

Whatever happened to the One while Sauron was in Numenor. He had it by then, I seem to remember (this part of the history always gets muddled in my head)?

He had it with him. It was one of the weapons that let him corrupt and control the hearts and minds of the Númenóreans so easily. Letter 211: "Sauron's personal 'surrender' was voluntary and cunning: he got free transport to Númenor! He naturally had the One Ring, and so very soon dominated the minds and wills of most of the Númenóreans."