The Duramax has a mechanical cooling fan because they're cheaper than an electrical fan.
Not really. The OE electric cooling fan for the V6 can be had for $189 (list price $349). The computer-controlled viscous fan clutch for the diesel is $252 (list price $465). It's not one of the fan clutches from the 1970s.
This is also why the aero shutters weren't packaged with that engine.
The shutter assembly doesn't fit on the diesel, the intercooler is in the way. Why would GM put more expensive parts on the cheaper engine and cheap out on the more expensive engine? That doesn't make sense, either.
Additionally, the speed of an electrical fan can be more fine tuned to match the cooling needs of the vehicle at the time, especially at idle. The greatest challenge of a mechanical cooling fan is A/C performance at idle in high ambient temps, the idle speed can be increased to get a higher fan speed, but only so much. Everything should be fine if the engineers size the cooling components appropriately.
With the diesel's mechanical fan being computer-controlled the computer can and does engage the clutch to increase airflow at idle for improved A/C performance. Similarly, the computer can vary the clutch engagement to provide similar control of fan RPM like an electric fan, but yes at idle there is only so much it can do since unlike an electric fan it can't run at high speed at idle.
As far as the air dam goes, the tech was a little right. The air dam's main fuel economy benefit comes from keeping the incoming air off the underbody of the vehicle, as it's not very smooth or sealed up down there. Air from the engine bay can escape via the wheel wells or the under body (the cowl is sealed because that is where the HVAC pulls air in) and the air dam can help with this. With the vehicle's forward momentum, the air dam can help increase the density of the air in front of the vehicle and direct more air into the grills, thus increasing the efficiency of the cooling stack. This is why most trucks are squared off with up-right grills. As previously stated, you'll only see a difference during extreme towing circumstances with the air dam removed.
Close. The air from the engine bay is both pushed out by airflow through the cooling stack as well as sucked out by the low-pressure area created under the truck, which the air dam helps to create. There's no escape out through the wheel wells, they are pretty much sealed off by the fender liners. The air dam doesn't direct much, if any, additional air through the cooling stack as it would have to travel up and around the bumper first. Air, like water, takes the path of least resistance. The air dam directs airflow to the lower side of the truck rather than under it which would be more turbulent. The reduced airflow under the truck while traveling down the road creates the low-pressure area that helps pull hot air out from the engine bay, much like what the plastic engine shield that is also under there does (It doesn't just protect the engine).
This is from GM and talks about the air dam and other aerodynamic designs of the truck, the same principles apply to the twins. There are also wind tunnel pics showing what the air dam does.
Hope this helps!