It is the duty of an officer to question if an order is “legal” or not. They are not robots, and they are not going to blindly follow any order that they think is questionable.... IE indiscriminately firing on US civilians, shooting down known allies, etc.
The perfumed princes of the five sided puzzle palace may be all “Yes Sir! Three bags full sir!” in regards to EmpERROR Zero, but the field commanders (who are the ones that matter BTW) are nothing like that.
You aren’t telling me anything I don’t know first-hand. Being ordered to shoot down any foreign warplane is not nearly as close to a possibly illegal order as being ordered to fire on unarmed civilians of one’s own country, so that’s not a very good comparison. Add to that the speed with which events happen in the air and the potentially lethal effects of hesitation, and chances are that young aviator is going to execute that order and ponder its legality or morality later.
I certainly don’t agree with it, I think it would be a goddamn disgrace to attack an allied aircraft on a mission that does not endanger US interests, but I think it would be very possible under the current circumstances.