Since when was the question ever about the "legality" of Mutiny......... it was never about if Poe was "legally" right for instigating his attempted Mutiny, but weather it was "justified". And yes, he had loads of reasons to justify his mutiny.
Also, I really don't think you want to go down that particular road, as your argument of "disobeying orders is illegal" is literally a hairs breadth away from the kind of "but I was just following orders" defense most of the people involved in the Nuremberg Trials attempted to use to justify their crimes.