The Military uses of the Palantiri.

[Inziladun]

Alfirin, I think you are correct in your comments regarding the Stone of Amon Sûl. The UT essay on the Palantíri says in Footnote 16:

The Kings of Arthedain....maintained a special warden at Amon Sûl, whose Stone was held to be the chief of the Northern palantíri, being the largest and most powerful and the one through which communication with Gondor was mainly conducted.

As for 'eavesdropping' on the Elendil Stone at Emyn Beraid, that could not be done. By themselves the Stones did not transmit anything. It needed a surveyor at the other end for any connexion to be established, and there is no evidence the Elves in the region ever used the Stone. It does appear to be a 'one way' Stone in any case, only looking to Eressëa (and the Masterstone of Avallónë, according to the Silmarillion).