You Asked - I waited a day or so before response, because people were talkative. 
Also, HERE BE SPOILERS, so have a care if you haven't read all the way up through Stormed Fortress.
For Caithwood: Did Asandir have grounds and permissions. Yes. On Three counts, all in plain sight in the book.
Grounds: he states his authority "on grounds of the compact" - what occurs on Free Wilds territory is not up to mankind's prerogative. The Paravians are absent, therefore, responsibility and surety for mankind's acts there fall to the Sorcerers.
Asandir issued a direct warning, long in advance: to the captain who argued with him during his first confrontation after landing: (The camp trafficking in slaves) The Sorcerer declared, exactly what would happen to any "two legged inhabitant" who lit fires or bore killing steelā¦he announced he would be waking the trees.
This warning reached Commander Harradene, in a dispatch sent by the captain, giving reason for his camp's retreat to Valenford. Harradene was told in that report that the trees would be wakened by Fellowship action - the commander chose to scoff, and went through with Lysaer's ordersā¦the scene is right up front, before the command to set fire was sent.
The direct warning, flouted, would be accounted a permission within territory where the Sorcerers held lawful authority.
With regard to Dakar's later handling, by Davienā¦the connections are close to debatable, but the verdict against would be sticky. Fellowship Sorcerers ALWAYS respond to the practice of necromancy, in one way or another. There are refs. all the way through, demonstrating this. In town territory, it's handled one way (due to the compact) in Free Wilds, quite another. In the chartered towns, quite another. See the appendix in either TK or SF, depending on your edition, for a clarification of how the law operates.
Etarra is not a charter town, or a recognized town, on designate territory.
Davien at the hour, was NOT in collaboration with his peers - he's known to act controversially. And tread a fine line.
His "grounds" to act - rested on two points, both on "thin ice" so to speak: Arithon gave his blood oath to survive to the Fellowship via Asandir. Davien would have chosen HIS WAY of interpreting "Arithon's survival" - and Dakar was only a spellbinder. Technically, he was NOT acting under either Sethvir or Asandir's direct orders; and he held Arithon's own permission - which, to Davien, constituted a problem, since an inopportune intervention by the spellbinder could have caused a conflict of interest, if not messed up the outcome of a tactic IMPLIED - that Davien's conversation indicates Arithon had his own plan in place, beforetime. This tactic, worked out ahead of Dakar's tardy arrival, was best not distrubed, and Dakar made plain his intent to bull into the mix, anyway.
The compact, too, could provide a lawful right for a Fellowship Sorcerer to intervene - Etarra lying on a major Lane - an act of necromancy at that junction would have caused a disruption of resonance that was far reaching, and damaging to a lot of critical territory.
SOMETHING would have happened, had Davien not actedā¦the rest of the Fellowship expected to be called in to help, by Arithon; that this option did not develop - we don't know what would have occurred if Davien had not stepped in.
That Davien (and presumably Arithon) had other ideas, and collaborated ahead - the clue for this was the blind old man disguise that Dakar could not "see" through on his arrival. The working was done by Davien's artistry, and repeated via the embroidery on the cloak, in Stormed Fortress. Upon Dakar's arrival at Etarra, it is very apparent Arithon had been seriously busy, arranging to carry out his charge to defeat the cult. The reader, and Dakar, are never told the extent of the plan, because, of course, someone would have acted to stop the enactment.