My understanding of the palantiri is that anyone could use them to communicate with someone at the other end of the line, so to speak; to see far and wide where one desired is what required a strong will. The Numenoreans were noted for their strong will and keen intellect, at least in the better days of the 2nd Age, so several of them may have been able to use a stone in solitary pursuit of information.
I learned something interesting while refreshing my memory among my source references--the Noldor made a master stone that was kept in the Tower of Avallone. This would mean that the Valar had their own means of keeping up with current events in ME. Maybe that's how they were able to see Gandalf after his Balrog battle, fix him and send him back so Gwaihir could pick him up and take him to Lothlorien!
Hmmm . . . if the Valar could see what occurred in ME . . . ah, but that's another thread for another day!
Also, none of this came from UTs. I, too, will be doing a more thorough reading of that once I'm done with
The Letters of . . . .