Gandalf, Moria, and Arrest

[Alfirin]

Originally Posted by Moonraker

Boromir was another matter, he tried to steal the Ring from Frodo, and theft of this magnitude does carry the penalty of jail in a civilised world.

Again, under whose laws? The attempt is made in an area without any local
people so not them.

Tecnically, Parth Galen is part of Gondor's Territory, and as such under Gondors law. And at the time of the attack, the ultimate arbiter of that law is Denethor, NOT Aragorn.If any question of crime arose, it would have to be Denethor, not Aragorn who would have to hear the case and pass judgement. Aragorn isn't king yet. Even once he became king, even if he did decide it was worth trying (which he probably wouldn't) he can't; few, if any, civilized legal systems allow for punishment for retroactive crimes (even if something is declared illegal now, you can't punish someone for doing it in the past, before it was illegal.) I will reiterate what the others have said, any decision by the rest of the fellowship to let Gandalf, Aragorn or anyone else be the leader, and to abide by thier decisions, was purely VOLUNTARY. Aragorn had NO legal authority to enforce any decision he made as law, heir to the throne or not.
Under the law as Denethor would interpret it, Boromir trying to take the ring would not have probably been considered a crime. Quite the opposite in fact, Denethor makes it pretty clear that in his book, NOT trying to take the ring and taking it back to Minas Tirith would have been the criminal act. Faramir makes it pretty clear in his words to Frodo in Ithilien that, by deciding to let Frodo and Co. go on thier way as opposed to taking them all back to Minas Tirith to see what Denthor thinks, he is techically comitting treason, and he damn well knows it.
As for trying to have him arrested in Rohan, how, the crime was not on Rohan soil, nor are any of the parties involved citizens of Rohan. Rohan literally would have no jurisdiction. And even if it did, it would not be much of a case. Remember the crime is only of great magnitude if you KNOW what the ring is, which no one in Rohan would. Short of someone in the party letting Theoden or Eomer in on the secret would would be dumb, especially Theoden (since, at that point Theoden would still be under Grima's ministrations so anything Theoden knew Grima would know and pass on to Saruman) it's just attemped theft of one gold ring. That's a minor crime at best, and given how well Boromir is thought of in Rohan, likely to be pardoned almost without thought.
To be honest, at this point, I think you are trying to cast Aragorn as some sort of absolute tyrant, doing whatever he thinks is best without council, and arresting or punishing anyone who disagrees with him. If he really WAS that kind of a person, there is no way he would have been allowed to join the Fellowship; it would be like letting Sauron himself join! From the beginning we are meant to see Aragon as a "good" and "wise" king to be, and a good and wise ruler does NOT just do whatever he wants. He may be the ultimate arbiter of the law, but he is NOT above it. To borrow a line from Tolkien's Friend C.S. Lewis's work "The King is under the law, for it's the law that makes him a king."