@axxe Happy to explain my thinking to you.
To start off, these are MY "standard house rules". I am sure other people agree/play by the same standards. Others probably do not and that is okay.
About the defender in a battle not targeting the attackers setup zone. These maps have a size limitation in order to make them playable. In many of the maps the setup zone for the attacker is quite small. Often a thin strip along an edge of the map or even just a small corner section. That zone is also very well known by the defender, since it is where their larger defensive setup zone is not, or they have seen the map from the other side and know exactly. This area is where the entirety of the attacking force starts the battle, which can range from a couple platoons to multiple companies and vehicles depending on the size of the battle. I think the attacker should be able to recon the battlefield, make their initial advance, hold troops in reserve, without the fear of getting blown off the map by gamey superpowers. In actuality clustering of troops in that density, in a location known by the opposing side just would never be a good idea, yet the map and game limitations force it. The attacking force cannot withdraw, fallback, take cover, or otherwise hide or maneuver out from enemy fire, even if that would be the prudent thing to do. So if the defender was able to target that small area full of clustered attacking troops I believe it gives the defender too much of an advantage, and an advantage that is based on game limitations and not combat tactics, making the game (in my opinion) not fun.
Want to know why I say no pre-planned arty/air at all from the defender?
Read this.
A few examples, first based on your question of the defending force penetrating the attackers setup zone on turn 5. I would say that would fall under the defender taking advantage of the game map, instead of having a more tactical fight on the battle map. Why should the defensive force be allowed to rush troops forward and try to score fast knockout blows, when they know they have the attackers full force pinned against an map edge or corner with no space to maneuver or fall back? To me that just doesn't seem right. The attacking force should be given the opportunity to find the defenders position and attack the objectives. And vice versa the defender should have to defend those objectives, which can easily be done without targeting the attacking force in their setup zone. This in no way means the defense can not counter attack, there is a whole maps worth of space for that, just don't target the attacking force in their setup area. For example, if my rule was not in place and American troops were attacking a German occupied area. The German player could spend all their purchase Panther and Tiger tanks, place them as far forward as possible in their setup area, immediately drive them to where they know the attacking troops are clustered and obliterate the attacking force. In my opinion that is just not fun or the point of these games for me.
My "don't target the attackers setup zone" covers it all. No loopholes, no "what if this what if that" scenarios, no "well maybe the attacker should have moved faster". Covers pre-planned arty in the attackers zone, covers trp's in the attackers zone, covers the defender advancing on turn one into the attackers setup zone, etc. It's simple and I do not think it takes anything away from the ability of the defender to defend their objectives. I have won many defensive setups following my own rules, and that includes mirrored games in which I won both games. So it is not some rule I made up to give me advantages when I play as the attacking force. (Although I did once have an opponent tell me I created a set of identical mirror games and forced him into these very rules which in his mind gave me all the advantages from both sides, even though we played each side, by the same rules, in the same exact conditions)
I have experience when the defending player pre-planned massive artillery bombardments into my setup zone as the attacker. Again I do not think this makes the game fun. The defender knows exactly where the attackers troops are, whereas the attacker has very little to no specific info on the defenders force at the start of a game. In this circumstance the defender told me I should have rushed my troops forward out of the setup zone and into buildings (exposed death traps he had targeted), without doing recon, without setting up a base of fire, etc. I do not think the only options given to the attacker should be to stay in their setup zone and get bombarded or to make a rushed and sloppy advance into the waiting sights of the defender. So that is why I have that agreement as a standard. This still goes for 5, 10, 15 minutes into the game, and continuing that throughout the whole game makes sense to me.
A few things the attacker could use his setup zone for other than staging troops... Having their on map mortars setup and make fire missions from the setup zone. If the attackers setup zone contained any point in which their troops could engage or target the defenders position, then putting units in that spot would not be using it for staging. It forces the attacker to leave their setup zone and attack, which is what the point is, but allows them to do it on their own timeframe, which is how attacking forces plan. I also word it how I do because people always try to find loopholes with things, so I think saying that the attackers setup zone will only be used for staging is a nice juxtaposition for saying the defender will not target the attackers setup zone. It's simple, thorough and in my opinion makes games more balanced and enjoyable.
Of course some caution and prudence should be taken by the attacker in setting up their force and looking about to see if it's in obvious LOS to the defenders possible positions. In my opinion good maps will make most of those problems irrelevant, but I have enough experience to know there are always exceptions and always players looking to take advantage of whatever they will. I think my standards are simple, thorough, direct, clear and make it hard to exploit loopholes.
Is someone does not like those standards they don't have to play me.
Hope that helps. - WS