Yes and no.
There area few reasons for the fuel efficiency advantage of diesel over gas.
#1 is that to generate the high compression needed for ignition, diesel engines have long stroke to bore ratios and run at lower RPMs giving time to extract more usable energy each stroke.
#2 is that the diesel throttles by varying fuel charge, so its almost always running over lean. However, have you ever seen a diesel under high load "rolling coal" ? Whats happening is that the fuel charge is actually over rich and some of its going right out the exhaust as soot.
Conversely, a lot of gasoline cars stay in the closed loop map at the 14.7:1 optimum lean ratio unless you either floor it to WOT or it detects detonation/high EGT then it will switch to the open loop map and go over rich to like 8:1
#3 And this is the big advantage - because diesels throttle by fuel there is no throttle valve and the intake is always wide open eliminating most pumping losses. This is an advantage to diesel at light load, but under heavy load when the gas cars throttle is open the advantage is gone.
So i think ?? the diesel overall might actually have more advantage at light load.
First of all, yes, I'm familiar with diesel engines, and gas engines, both in the engine/dyno lab and on the street (and off-road). I understand the primary differences between them. Your theory's are a bit flawed, primarily because they are just theory and don't actually take into account all the variables.
#2, "rolling coal" has nothing to do with this. If some punk young kid wants to chip his truck and pour tons of fuel into the cylinder to produce soot it has nothing at all to do with fuel efficiency. We're talking about reasonable use of a truck. Yes, there are occasions where even a factory truck can produce some soot, but the turbo charger does a pretty good job of taking care of that issue. Within normal operation the diesel engine, even heavily loaded, won't produce soot, and will run very efficiently. Why? Because it has a turbo. The turbo pumps additional air into the cylinder to make up for the additional fuel being supplied and you get tremendous torque rise. You won't get that in a gas engine, to compensate you must go larger engine. Of course, you can turbo-charge a gas engine too, but at high boost pressure in a gas engine you have to richen the AFR quite a bit to keep combustion smooth and prevent overheating of engine parts. It's just the nature of the beast.
#3 First of all, I think you have a mis-understanding of pumping losses and how they effect an engine. The more air available, the more air the engine ingests the greater the pumping loss. That being said, pumping losses are mostly negligible in the grand scheme of things. We're talking about towing a heavy load, pumping losses only make a noticeable difference to the engine (gas or diesel) at light load. You stated that the diesel loses it's advantage when the gas is at WOT, but remember the point you made in item #2, at WOT the gas is going over-rich, and efficiency drops off a ton. Whereas at WOT, the diesel is still running at/near optimum AFR and the only downside you highlighted was that it loses it's advantage on pumping losses. Even if there were an advantage is it really a downside that the diesel "lost it's advantage" on a negligible point? The real magic is happening with the AFR and the torque rise due to the longer stroke of the diesel, not the pumping losses.
Throw out theory and "what should happen" all you want, but hook up a gas truck to a the same trailer/load as a diesel truck, run the same course of driving, then do the same with no trailer and compare the results. Diesels have an enormous torque rise when they are loaded down and can still maintain an optimum AFR, gas engines just can't match it. You will see less fuel economy drop on the diesel engine when it's towing a load than on the gas...
