[NoSanninWa]

It is unknown if an open ended order can be give like "obey any orders I give you." So far no such orders have been so vague. Perhaps it doesn't work or else he'd do that a lot. My personal guess is that a geass needs to be partitioned in a person's mind and an order like this is too open eneded to be partitioned.

However, returning to your first point, geasses are definitely not structured. He gives simple orders and the target figures out any necessary steps on their own. For instance, he says "Let me go with all your strength" and Orange-kun first gives an order to let Zero go and when it doesn't work, he then decides to attack anyone who tries to stop Zero. This means that every order involves the decision making ability of the victim.