All you have in <HOI2 is groups of divisions moving around a map. You can hope for operational command structure and I try to simulate it as much as possible, but it is barely represented. HQ helps. The only reason I group more than 1 division together is for ease of movement and my preference. In <HOI2 is does not really impact a battle if you are 5 separate divisions in a province with an HQ or 5 divisions stacked with an HQ. Except that you get the benefit of each divisional commander. The computer creates a glob of divisions sometimes, but it has the luxury of being all-knowing. As the slow human, I have to locate stuff easily to whoop ass. That is not why divisions group themselves into higher command structures in real-life, as you seem to know.
According to the dev diaries, HOI3 will allow formal corps/army/theater unit structure. I believe and hope this wil be more of what u speak.