[Belegorn]

Originally Posted by Ivriniel

So, Elendil places the Master Stone in the Southern Kingdom, not the Northern Kingdom, with Anarion overseeing that, although Elendil, we're told, assumed reign from the principal kingdom 'of the North'.

From Of the Rings of Power and the Third Age, "Three Elendil took, and his sons two each. Those of Elendil were set in towers upon Emyn Beraid, and upon Amon Sûl, and in the city of Annúminas. But those of his sons were at Minas Ithil and Minas Anor, and at Orthanc and in Osgiliath." [p. 362] One thing of note is that the towers of Emyn Beraid "were raised by Gil-galad for Elendil". So there are parts of Arnor that were not built by the Númenóreans at all.

Originally Posted by Ivriniel

This seems to me a problem. How does the primary overseeing realm get reconnaissance from the Amon Sul stone, where that can be considered wholly reliable, where the possibility exists that the Southern Master Stone can shut out communications to Amon Sul?

It's more like if two stones are in communion, say the Orthanc and the Ithil-stone, any other Stone would see them as blank. The one at Osgiliath of course did not have this limitation. So it can't shut out communications, rather it can eavesdrop on two other stones in communication. Gandalf himself tells Pippen, "at Osgiliath they could survey them all together at one time." [TTT, Bk. 3, ch. 11, p. 240] It is also said how Denethor and Saruman could have had communion between themselves without Sauron being able to see what they were communicating. Between them they were working the Arnor, Orthanc, and Ithil Stones, none of which can do what the Osgiliath-stone could. If Sauron possessed that, he'd be able to see what Saruman and Denethor were communicating about.

Originally Posted by Ivriniel

There is, at best, a reliance upon trust to ensure fidelity to sovereignty matters, and at worst, the potential for insurrection, covert, Machiavellian events and deception.

At first, at least, they were in the possession of Elendil's sons. Did a High King up North figure that maybe he should move the Osgiliath-stone up North so that they alone could look into the conversations between other Stones? I do not know.

The Stones could be a drain mentally and usually the Kings had people below them make use of the other Stones to scan the realm so they could be on top of any potential enemies and to communicate. In fact, remember the episode between Arvedui and the Council in Gondor when he claimed the High Kingship? This was more likely than not done with the Stones. In Note #1 of The Palantíri:

"Doubtless they were used in consultations between Arnor and Gondor in the year 1944 concerning the succession to the Crown."

Originally Posted by Ivriniel

Unless, that is, the Amon Sul Stone had other properties we have not been told about.

Except for it being like the Osgiliath-stone, minus the eavesdropping capability, it would be more powerful than the other Stones.