Thanx for the knowledgeable answers.
I suppose you can represent this by, as said above, making small divisions consisting of two or three brigades and by appropriately renaming your HQs and units. There should, for example, be no problems in naming a corps an "Army" or an army or army group a "Front" etc.
As for the divisional/corps/army support units they simply have to be understood as abstracted in the brigade stats and doctrines of the respective country. I think this is next to necessary as well as anything else easily could turn into micro management hell. Though I'm sure there will be mods for those who just can't get enough.
