"I'm not clear why anyone thinks that if I send a JAP Infantry Army with 10 Divs into North China, that I must have 2 or 3 Corps commanders/HQs well as the Army."
Because I can group my armour into one of the Corps and achieve a local superiority in numbers or type. One the defensive I can assign a Corps to defend a province or two and would hope that my Corps commander will rotate his units and or keep a reserve in case a primary assault happens on his patch. The alternative is to have an Army commander with a mix of divisions, which will be spread relatively evenly. This is easily beaten by concentration and specialism. I can assign a mountain Corps to a particular sector knowing the terrain, an amphibious Corps to take an Island ( this worked remarkably well in hoi3) or a line infantry Corps heavy on artillery and engineers to take forts.
You would end up micromanaging 25 divisions in an Army, or ending up with all sorts of units in the wrong places, I could achieve local superiority and smash your lines by merely commanding 5 units.
"Or why Rommel can't command the DAK on his own, without any lower levels. Nor why we must assign an Army Group and Theatre commander above Rommel, and have their rear echelon HQs on the map."
Rommel did so bad example. Group and Theatre were pointless in hoi3 other than a strange bonus system. Don't confuse this with being pointless in real life though. ... [snip]
Don't confuse hoi3's system with a proper OOB.
I used the example of Japan, an Infantry army, North China. What armour would you be grouping into a Corps? What mountain corps and indeed what mountains? Which Island would the marines be taking?
And I would be beaten by exactly which specialism of the Chinese? Those specialised militia really fill me with dread.
The only "specialised" unit in the JAP OOB are the Mongolian Cavalry - useful in the remote hills inland along the Mongolia border, where IC is extremely low. By mid-1937 I might have created an Armoured Div with light tanks. Put it in one of the attack vectors to give them a bit of a boost in the attack stats, in the centre or left flank.
Strategy involves managing the supply situation, winning with minimal units - if you pile in too many units, then you run out of supplies inland. Capturing the port near Beijing is a necessity. Being very careful with basing air forces, keep them out of supply lines in North China as long as possible. TAC can fly from Dalian, though they won't be able to cover whole area of the invasion. Use CAG from a CV task force in the Yellow Sea.
Capturing the port at Tianjin is important to improve supply lines. It can be captured by ground forces, but you might want to make this as quick as possible by an amphibious invasion. Not by a marine Corps though. IRL JAP landed the 5th Infantry Div there. But by the summer of 1937 I probably have enough new Inf Divs built on the home islands to launch a corps sized invasion, further down the coast behind Chinese front line, diverting forces away from Beijing. But you have to be confident to push down the coast from Tianjin to link up with the beachhead, so they don't run out of supplies. Or you delay the invasion, and send a larger number of Divs to the peninsular further south, and capture the port there. This then gives a major staging post to push inland, and split Chinese forces.
So however you do it, two phases: Beijing and Tianjin; defeat main forces, then keep them on the run, with three ground attack vectors for the ground forces - into Shaanxi/Inner Mongolia, Shanxi towards Yuncheng and down the coast. And you might have an invasion vector. Pause and create a separate battle plan for a major invasion either the peninsular or at Shanghai. Capture Shanghai and you could branch out in a number of directions, but a couple of attack vectors are the way to go - north along the coast and north west. Keep them in touch, aim to link up with North China forces. If you can emphasise the inland vector, include some armour if you have it. Try to pocket a major group of Chinese forces between them and the coast.
This is all going to take a couple of months. There is no need to have a single very complicated battle plan with several phases and many branching vectors, including the invasions. You might not have most of the Divs even built before Marco Polo. The additional IC available once DoW happens quickly allows major expansion of forces.
BTW I never mentioned about the usefulness of higher level HQs IRL. My argument is these levels above the level of combat, add nothing to the gameplay. As soon as you decide which level of formation you are going to be using as your primary combat forces, anything above that is pretty irrelevant. If you need some sort of bonuses affecting rear area forces, logistics etc. then using the General Staff guys to decide what bonuses should be sufficient. Have someone as a logistics wizard in the General Staff, rather than an Army Group commander would work for me. So unless the devs had a plan for the higher level HQs to have a very specific logistics role, such as operating as a mobile supply base, then they can be removed from the game. If the devs give us a good way to control build-up of supplies for a major offensive that doesn't depend on any on-map HQs then that's great.