First, I'm glad you found a nice car that makes you happy. Second, I'm glad you're interested in how it works, and how to make it better.
If I may opine on EGR; ambient air is made up of about 20% oxygen and 79% nitrogen. Exhaust gas is made up of oxygen, nitrogen, water vapor, and carbon dioxide (mostly). To burn a fuel it takes the hydrocarbon (HC) fuel and oxygen. The chemical oxidation process reduces the HC/O2 to H2O and CO2 + heat. Now if you were to pipe bottled oxygen into an engine with a stoichiometric amount of fuel, it would burn, but the engine would have no power, and it would start melting parts almost immediately. Why??
The nitrogen in the air acts as an "Expansion Medium" which
absorbs the heat, and in turn expands (creates pressure). Some gasses are "acceptable" expansion mediums, while others are stellar. Water vapor expands at 12X the rate of nitrogen with the same amount of BTU heat input. Carbon dioxide even more so. EGR is actually a superior
expansion medium to air.
The trade-off is exhaust gasses contribute very little to the oxidation process. A little EGR and the engine runs a bit better. More and it runs even better. More and it starts getting sluggish, and HC/CO emissions start to spike. The flame propagation process in inhibited by the lack of oxygen.
So how does EGR reduce NOX? The more powerful expansion medium is able to absorb more heat at any given volume. NOX is formed by
temperature over
time. Reduce either and NOX goes down. Since the more powerful expansion medium (exhaust gasses) are absorbing more of the heat of combustion, peak cylinder temperatures are reduced; thus reducing NOX.
George Arlington Moore took EGR to radical extremes back in the 1920's with impressive results -- both fuel economy and emissions. Here is a good primer on
Combustion Efficiency.
As for fuel economy mods on diesels, the largest gains I have been able to squeeze from them is with ozone injection. Whereas ambient oxygen is O2, ozone is O3, O4...O60. It is far less stable than ambient oxygen, which reduces the endothermic losses associated with flame propagation, as less energy is required to split off oxygen atoms.
Ozone can be made with high voltage across 2 stainless steel screens separated by a dielectric. This method is common in air purifiers. Another is with a spark gap. A gasoline engine distributor cap, brushed DC motor, anything that sparks generates ozone. A 3rd method is with 185 nm UV light. (Unfortunately, unlike IR spectrum light, there are no LED lights able to reach the 185 nm range. These lights are all mercury vapor.)
The second best gains was with HHO --
copious amounts of HHO! In my initial testing I used about the same amount of HHO I would for a gasoline engine of the same size. The problem is that at say 2000 RPM moderate load on a gas engine, the throttle plate limits the amount of air in the cylinders to maybe 20% VE. The diesel is gulping 100% VE all the time. So the air-to-fuel-HHO ratio that works on a gas engine simply was nowhere near enough HHO for the same sized diesel. When I cranked up the HHO, things started to look brighter.
OOoooohhhh, look at the time! I think I should stop now. I hope this gives food for thought.