Gandalf, Moria, and Arrest

[Belegorn]

Originally Posted by Moonraker

Moria was a venture into the unknown,

Why would Aragorn even contemplate arresting Gandalf when it appears that going through Mordor was not out of their equations anyway?

Originally Posted by FotR, The Ring Goes South

not until it is plain that there is no other way.

They did not know who or what was there, be it friend or foe. The impression you give is that they should never go there, and that Gandalf would have been forbidden to venture there by the Council, as if they knew they would encounter a Balrog.