LotR -- Book 3 - Chapter 11 - The Palantír

[Neithan]

I don't think that the Orodruin had anything to do with the secret fire, Eru would not have allowed that. It was more than just a volcano, it was the hottest place on earth. The Ring was so indestructable partly because it had been made there.

The Ring has this 'dual' aspect - it exists as a physical object & as a locus of spiritual evil. The physical fires destroy the gold, the spiritual fires unmake the evil.

The Ring did not have a spirit. It is a tool used for a specific purpose, powered by 'magical' energy, and given an AI-like consciousness. Thus there is no need for spiritual destruction and it makes perfect sense that it could be destroyed at Orodruin. As far as the Palantiri are concerned I think you are overthinking it. Mount Doom was the most destructive place in ME so if they could be destroyed anywhere it would be there and they were just not made to withstand that kind of punishment.

As for the 'addictiveness' of the stones I think that it was Sauron. Sauron took the stone originally because it allowed him to do something that he could not do before. It allowed him to see afar, this became his 'eye'. There was another benefit that he gained from the stones however, through the 'network' of the Palantiri each stone was connected to him. It is through this connection that the temptation comes (beyond the natural temptation of being able to spy on everyone of course).