[Imp]

What I was refering to with Z fire at a hex you cant see is it normally passes through a dense hex like woods or buildings. The lighter the round the more susceptable to richochet even foilage will deflect it let alone hitting a tree.
If you fired into woods what percentage of those rounds do you think would miss all the trees & make it to where you are aiming if you cant see it. Most would be deflected or stopped I feel.

I am classing APCs as a heavy weapon simply because they can maneuver to bring multiple units to bear even if they dont drop passengers. They can help force route by causing encirclement if the unit is now surounded on several sides. Like I say try it without vehicles.

The distance is 50m NOT 100m hex centre to hex centre not edge 100m is fire with a hex seperating the two hexes. Call it 30-70m if you like 80m is the next hex.
That being said 50m still offers plenty of cover in an urban setting part of what I think you are after may be due to scale & generic urban hexes.
Concievably 2 squads in the same hex could easily still be in diffrent houses or 2 squads in adjacent hexes could be in the same room say a Factory floor with few lines of sight into from outside. Perhaps this is what you want a building that has a covered arc so can only fire from the windowed side. Should only be able to enter/exit from there to unless you make a hole. Not ideal but try using a bunker as a sort of seperate room.
Its a question of scale & a very old game engine you should probably not really be able to fire that RPG & certainly not an ATG in a 360 degree arc from a building as probably need to change rooms. Now you have a reason to go room by room if the scale allows for doors windows etc.

I need you to explain about holding the block & fighting on equal terms how you think this changes things as I think keeping people out of your block is important anyway & checking its clear is important before moving on.
As the game works now 3 things normaly happen when adjacent in a city
1) Defender in building attacker in street probably moving.
Good cover vs very poor cover & moving - big advantage to defender.
2) Both units in building attacker probably moving - slight advantage to defender
Hence keep him out of your block as stated.
3) Enters enemy hex, if defender has shots this is nearly identical to 2 though the defender has a chance to suppress or hit himself. Several men in a confined space stray shots.

So if he stays how does a unit entering its hex benefit the defender? As you said want to stop him in the street once he is in your block you are in trouble. This is what I am struggling to understand how it would change things.

Also perhaps try playing on a pregenerated map where you can often give more covering fire to other blocks as they are not laid out American style but with windy roads & smaller blocks so stopping in the street tends to be easier.
This will not really help the AI as a defender in fact probably the reverse as you take advantage of the aditional fields of fire but it helps a human defender.

Another thing to try to achieve staying put is build fanatics say exp & morale at 100 but I would adjust Inf command or they will be crack shots. I think these will stay put longer.

Some factors that might cause a unit to run.
Being fired at especially from multiple directions or even seeing you are surrounded.
Outnumbered the closer they are the bigger the threat.
Freinds killed or decide to run.
What causes him to stay is the reverse so if the street is litered with the enemy & burning APCs then despite all the above he still might stay.

What situation do the defenders in your games see?