Yes, that is true, the dammage inflicted at all angles is the same, but simple dammage isn't all that happens! Tanks can be immobilised when their tracks are dammaged, their turret can be disabled, and of course that little picture in the top left of the hud shows that the tank can register rear dammage as well as dammage to the main chassis. Just what operation flahpoint does with these values, I can only guess at, but it does something....
As for the real law script, I don't think it is as realistic as it could be; it doesn't stop dammage accumulation, but it gives certain types of ammo a higher probability of killing the tank by giving the occaisional 'instant kill'.
I think that the process is needs work in a couple of places:
1- you have to have checks for all possibile ammos that could dammage a tank, this is bad if the user has a custom law soldier, etc etc etc.
2- That you have 4 different scripts that have practically identical bodies, apart from the line
_Shell = NearestObject[ _tank, "CarlGustav"]
in which only the ammo name is different. They could all be compressed into one, easily. I realise that if OFP was more editor friendly, you could pull up values for dammage and type (AT/non-AT) for any kind of ammo, and make your decisions based on that, but unfortunately there's no "ammo.dammage" or anything like that.
The reason I suggested the combination of (custom tank/high armour/custom ordinance) is that:
1- Armour is high so that nothing can instantly kill it (e.g. lgbs, mavericks). This gives you time to reduce the dammage if you decide that the projectile which just got fired at your tank is going to bounce off it or something. No good repairing the tank once it's dead, as the crew will be killed.
2- I used custom ordinance so that if you decide, for example, a rogue LAW has hit the tank at just the right place to do a critical hit, you are able to dammage the tank (don't forget it has a high armour value), AND (this is the important bit) take advantage of BIS's existing dammage system. This especially is where my method differs from yours. With your armour system, 4 laws will always kill a tank, but sometimes 1 will get lucky and kill it straight out. Under my system, 10 rpgs could be fired at the tank's front armour and not scratch it, but one fired at it's tracks will disable it, every time.