This does seem like a nice idea and also really simple which is good.
Should there be some differentiation between number of concurrent orders for different types of ships (ie fighters vs cruisers vs frigates etc)? Skills or officers could modify this I suppose.
Another approach would be that each size type of ship (frig, fighter, etc) requires a certain amount of time to 'recharge' before that ship can accept a new order. This may sound kind of stupid at first, but consider: fighters can quickly change orders and re-engage targets, but for a capital ship, first you the fleet admiral have to contact the captain of the ship, then he gives the word to the first mate and bosun, then they give it to the various department heads, engineering is notified, new courses are set, etc etc until it trickles down to the individual gunners and guys shoveling coal into the engines. So it would take a while before they can change the ship's overall task.
Then different subskills under leadership could reduce the 'recharge' times for different classes of ships, so you could micro more. Fleet officers who effect this kind of thing, if ever implemented, would be cool for that too.
But as it is there isn't so much tactics or real objectives on the battlefield so the frequency with which you need to make orders isn't very high, perhaps making this scheme sort of irrelevant.