I feel the big factor missing in the entire targeting scheme so far of front line attack/defense, support and rear echelon is mobility. Static guns and infantry on foot is very unlikely to provide a breakthrough and attack rear positions, while armored or air mobile units can possibly strike deeper.
Similarly, how does fortification interact with the positions? It does not seem like you should be able to fortify much in a frontline attack stance, as it suggests being on the move, outside of well prepared positions.
Only allowing superior formations to support is also kind of restrictive, what if you want to have two artillery battallions per 3 frontline formations, and support two of them? It also makes it impossible to have independent artillery formations that are not headquarters.
I would very much like to be able to shift whole artillery formations around between formations