You are all approaching this problem of the destruction of the Ring from the wrong angle. We don't have enough people here familiar with magical traditions, I see.
There are many traditions that feature the "separable soul" as a magical operation. Either as a means to protection -- the "person" whose soul is stored in an object often cannot be harmed while the soul is safely contained away from their body -- or to gain power -- of course when someone else obtains the object containing someone's soul, they gain power over whoever the soul belongs to/comes from, but also in operations such as the binding of demons or other spirits to service of a sorcerer. The Harry Potter franchise used this very recently and I'm a bit surprised that no one thought to make any analogies. The main villain could not be killed until all his various soul fragments stored in external objects (one of which was the
protagonist - an interesting twist) were somehow either destroyed or otherwise purged of his "soul stuff".
As usual, Tolkien adds new twists to the traditional motifs. First: Even though the "good guys" effectively have the "soul" of their enemy in their possession, they cannot use it to control him. Instead, if they tried they would be corrupted and become just like him, effectively being
possessed by him... The "soul" is too powerful to control even though contained in an object. And yes, that effectively gives Sauron a sort of "magical protection" against final defeat even if his body can be killed, as it was an age ago. Placing one's own soul in an external object is a common prerequisite for sorcerers to
possess others, as it happens. The underlying mechanic seems similar with the lesser rings; people who use them effectively become more and more aligned with the device (machine?) they are using until
it rules
them instead of the other way around. It amounts to a sort of "gradual" or "piece work" selling of one's soul to the Devil, instead of a single catastrophic signing off.
The Ring of Sauron is only one of the various mythical treatments of the placing of one's life, or power, in some external object, which is thus exposed to capture or destruction with disastrous results to oneself. If I were to 'philosophize' this myth, or at least the Ring of Sauron, I should say that it was a mythical way of representing the truth that potency (or perhaps potentiality) if it is to be exercised, and produce results, has to be externalized, and so as it were passes, to a greater or lesser degree, out of one's direct control. A man who wishes to exert power must have subjects, who are not himself. But he then depends on them. -- Letters, #211, p. 279
Reciprocity is a big thing in all mythology and folklore. From the doctrine of 'karma' in Hinduism to 'pacts with demons' in medieval magic. Middle-earth is no different, but Tolkien uses it in a slightly different way than has usually been done. He introduces reciprocity with the 'device' or 'tool' of power itself. The problem of the machine, as he calls it. But it otherwise follows the traditional rules. So it is not really necessary to find the user manual or capture Sauron's lab notes to understand the gist of the situation. While the current "Council of the Wise" comprised of both Elves and Maiar does not
know in detail exactly what the result of destroying the Ring will be, they can make a pretty good guess based on their own experience of how magic works in Middle-earth, which is more-or-less how it works in other mythologies despite Tolkien's talent for creative twists. They cannot use it against him, but destroying it has got to do him very serious damage because of how much he had to invest in it.
As for Elrond and Cirdan not grabbing Isildur and heaving him bodily into the Cracks of Doom if he would not let got of the Ring, I think the basic problem of means to an end is the correct issue to focus on. If they attacked him, how could that have a good outcome? This again goes back to a knowledge based on experience. It seems unlikely that Elrond has only recently begun to grasp the nature of those past failures to defeat evil. And it also seems unlikely that he understood that the issue was that urgent.
While I think he had to already understand, even at the time of the Last Alliance, that the Ring represented a significant amount of Sauron's power, I doubt he understood the
proportion of Sauron's power that was invested in it. Everyone was already aware that Sauron was a Maia and would be back sooner or later, despite having been temporarily deprived of a physical body. He was probably thinking that destroying it would weaken Sauron
somewhat, and that would of course be a good thing, but not that it would render him so dramatically weaker that he would become a shadow of his former self.
"For he will lose the best part of the strength that was native to him in his beginning, and all that was
made or begun with that power will crumble, and he will be maimed for ever, becoming a mere spirit of malice that gnaws itself in the shadows, but cannot again grow or take shape. And so a great evil of this world will be removed" -- Gandalf, speaking in The Last Debate (Ch. 9 of Return of the King)
At this point, Gandalf had been back to the home office for "consultations" so he probably has expert knowledge from someone (Aule?) who knows this lore better than anyone else around in Middle-earth right now. Without grasping how grave the damage to Sauron would be, Elrond would not have been over-the-top determined to force Isildur to destroy it. He would have regarded it as important, but not an existential issue. Sauron would be back sooner or later, but now would be a good time to destroy the One Ring and possibly make him weaker next time. Thus we have the story told at the Council of Elrond -- "We tried to get him to destroy it, but he refused"...
