Speaking of that, one minor thing that I didn't notice being explicitly mentioned (although I suspect you would have noticed when you got to it if you haven't already): if a formation is set as Support or Rear Echelon and is attacked, any element of that formation with direct combat capability ought to be able to shoot back at the attackers. Otherwise there wouldn't be any much point in attaching a platoon of infantry to guard your headquarters/artillery park/whatever. Whether bombardment weapons tasked to support should also be able to target formations directly assaulting them is arguably messier, but leaving it to random chance would create enough variation to tell all the different possible stories - shelling their proper target even as they're overrun, turning the guns on a small band of skirmishers while the army bleeds on the fortifications they were tasked to suppress...
While this would seem to make sense at first glance, it kind of messes with the implicit design detail that formations can be any size, and should function identically in combat regardless of size.
It's tricky to explain, but to give an example... let's say I have a bunch of front line units and some rear echelon formations with my STO weapons and such. Assume that the front line units mean each enemy formation has a 5% chance of attacking the rear formations.
Now, obviously the more formations the enemy has the more chances they have of hitting the 5% chance. But whether it's one enemy formation or 50 enemy formations, on average their weapons will be firing on the STO units 5% of the time. The enemy might think it was better to have 10 5% chances of doing 1 damage than 1 5% chance of doing 10 damage, but overall it isn't a huge difference
But if we assume the rear echelon formations can fire back at attackers, the reverse isn't true. If I have 5 formations in the rear, and one enemy formation hits the 5%, then one of those five formations returns fire. If I have 1 giant formation in the rear, and one enemy hits the 5%, then every single unit I have in the rear echelon gets the chance to return fire.
So it's probably better that rear echelon units can never return fire. If you want troops defending them, probably best to put them on the front line; this will both let them inflict casualties every round instead of just when they're attacked, and reduce the chance of enemy attackers actually hitting the rear echelon units.