I can’t pass up a straight line…
Of course we are - the Air Force recruits for intelligence, rather than brute strength. Guess where I served 
Couple of general points… the military is governed by three major rules of law - the Law of Armed Combat defines what can and cannot be done on the battlefield, the Geneva Convention does much the same but in addition controls how non-combatants, POW, etc… are to be treated, and the Uniform Code of Military Justice defines specifically the justice system for military members (as it is outside of the direct scope of federal / state law). Each branch of service has a subset of laws specific to that service, for example, how each service wears the uniform.
Mutiny is definined here… and it’s not just refusing to follow an order but refusing to follow an order with the intent of ursurping the relevant authority as part of a group.
One thing you are allowed to do in the military is refuse an unlawful order. For instance, you don’t have to kill children, and should an officer order you to do so, you can refuse. What you can’t do is refuse to follow an order because it puts you at risk of bodily harm. That would completely destroy the ability of the military to make war. I can imagine it now -
‘Take the Hill!’
‘No thanks, sir, that would be dangerous.’
‘Damn it, you got me there, I guess we’ll just sit here for a while.’
There are instances where mutiny is legal (or even your duty) under military law, such as pusilanimous conduct in the face of the enemy. In this case, it is actually stone cold legal for a commanding NCO or officer, or even a low-level grunt, to shoot those who try to run away, including superior officers, or to remove officers from command.
This seems to me to be an incident of soldiers illegally refusing an order, but not mutiny, but since they went outside their chain of command (called home) I think the soldiers who did this are in for a bit of a shit-fight, right or wrong. It’s not enough of an excuse that just cause they are reservists they’re getting the shit end of the stick when attached to a regular Army unit. Marines seem to always get the shit end of the stick WRT equipment and such in comparison to the other services, but they still follow orders.
Alessan - all of your questions are good ones, but bear no relevance to the legallity of these soldiers refusing orders. Just 'cause a mission has risk is not reason enough for the soldiers to question the orders given by higher authority.