You have to be your own judge. Role playing is all about adjudicating what doesn't exist and what lies between the rules. Your job as GM is to make the call for your party based on your knowledge of your players. If you think Rapid Reload would be good for Advanced Firearms, then give it a try...
There are two extremes in gaming. Simulation vs. Abstraction. You are arguing for simulation. The description argues for an abstraction.
One reason both barrels are clarified as being -4 is that there are some feats and defenses that can be only employed one attack at a time.
Thus while...
I surmise that they say "on both attacks" to avoid any confusion about the application of the penalty.
However the earlier text in the description makes it quite clear: "one barrel at a time, or both together as *one* attack." which would means you only have once chance to critical when firing...
$0.02.
Dangerously overkill for firearms. Be wary of munchkin Gunslinger builds.
It's better than "Rapid reload", and therefore should probably be +2 or +3 cost.
I would not allow it in my campaign.
The rules specifically say when firing a single shot from a double barrelled shotgun the damage is 1d8.
With a double shot you have lost the possibility of the second shot being a critical and incurred a penalty to attack.
With two shots you have a roughly a 10% chance of one of them being a...
You're a DM! You can do whatever you want, I'd say after 6 months of operant conditioning that your wizard will do whatever you want as a free action compulsion with a really high DC. There has to be a story behind the rules and while RAW says nothing about operant conditioning you can't argue...
Holding high-level casters captive is not that hard. If they don't have meta-magic simply take away spell components, or focuses (can't cast anything with DF or M), and then bind their hands and gag them? Or worse just keep them from sleeping or praying and they can't get their spells back? If...
I'd suggest something more subtle than just a combat. The desert is a harsh place and travelling caravans group together to ensure the safety of everyone involved crossing to the next suitable city. Why not have them encounter a caravan of slavers? The slavers though pose as a travelling casino...