Someone will answer soon I'm sure, but I have played HW solo and MP for almost THREE years and have never, even one single time, bothered to cancel an order and generally speaking I have less "order" problems than anyone else I play MP with - best keep things simple I say !!!!.
What I do is give a brand new order, what then happens is, the Corps carry on with the old order until the TIME arrives for the new order to kick in, then the troops start to carry out the new order. If you cancel a order, you have to wait to carry out a new order anyway, why bother ?.
I have always thought issuing orders, canceling orders, giving news one and them canceling them is not a great idea.
In my experience the LESS orders you give the better, giving new orders, or canceling old one every few minutes is not really the way HW was designed to be played.
Hope that helps a little.......just don't bother canceling them, issue new orders......ha.