Your question is naive.
An enlisted soldier (an officer, as well) is required to obey lawful orders from his/her superiors. A soldier is also required, under the Uniform Code of Military Justice, to NOT obey an unlawful order. However, at least when I went through basic training back during the Vietnam "conflict," the latter part was just barely mentioned, and it was not presented as a requirement. It was presented as if we could CHOOSE whether or not to disobey an order we knew to be unlawful. Now that I am older and (I hope) wiser, and have read portions of the UCMJ, I know that this is not optional. A soldier is REQUIRED to disobey an unlawful order.
The problem, of course, is how to know that an order is unlawful. You propose that the order to fire on civilian vehicles has come down through the chain of command and is, therefore, legal. Two problems. First, even though it may have come down from somewhere higher up, it still may not be legal. But the grunts with the boots on the ground may not be in a position to know that. Second, the grunts probably have no way of knowing if the order has come down the chain of command. If I'm stationed at Exit 78, I probably get my rules of engagement either from a squad NCOIC or maybe from the platoon leader, a 1st or 2nd Lieutenant. How do I know the order has come down the chain of command? And how high are you positing the origin? The theater commander? The Sceretary of Defense? The President? How high?
I'm really not sure, but I think in your first scenario I would fire.
In the second, we all know that the right to keep and bear arms is guaranteed by the Constitution. Therefore, any order to confiscate arms is unlawful because it is unconstitutional. I would not obey.