Three shots against three foes is fine. I add a penalty of -20 per target after the first (-20, then -40 for the third shot at a new target). Changing targets on the fly like that is difficult.
Yes, even with normal 2wpn combo, the off hand recieves a -20 mod, unless ambidextrous.
You can allow guns to shoot at targets with positional mods. At first I assigned a penalty equal to 2x position mod, but after play testing reduced it to a penalty equal to position mod (-15 for flank, -20 for rear). Shooters can spin around and shoot without changing facing, but they suffer the penalty for doing so.
Bullets cannot be parried. Only cover and Qu bonus count, possibly superior armor. There is a talent in Pulp Adventures that allows firearms to be parried. OTOH, a fellow GM allows parry if you have cover, up to the covers value (which is simular to the rules for cover DB versus supression fire, i.e. the cover value is doubled). It did not unbalance the game. Charging 50% activity sounds a good idea, as this will reduce the amount of firepower that can put down range, while having the advantage of increasing DB. Still, one three round burst at 50% will only suffer a -10 OB penalty and do shrapenel crits. Nasty stuff.
lynn