I'm not sure about your thoughts on the DEF tank and cabin air. It's clearly located outside the cabin just forward of the rear bumper below the spare wheel well. I can't claim to know this for a fact, but somehow I doubt there would be cabin air vents exiting under the vehicle exactly for the reason of introducing exhaust fumes into the cabin. DEF is also supposedly good to -12F so I don't think there is much practical reason to try to control the temperature of the fluid. The tank is heavy gauge steel, which you can see for yourself is you tap on it, so even if the DEF were to freeze, as long as it's not overfilled into the filler neck, its's not going to damage the tank. The odor from the DEF smells very clearly and strongly of ammonia. If you are smelling something that is not akin to ammonia, it's not the DEF.
Mine has done at least one DPF regen that I was aware of, and only because when I pulled in into the garage you could tell the exhaust was way hot and had a bit of a burning smell to it.
The other day when I was washing it I was noticing how the cowl vent is designed, just below the rear part of the hood. The way they designed, it, it's possible that there is a strong venturi effect there at the back edge of the hood and maybe that is why external odors are more easily picked up? Pure speculation, but made sense to me when I looked at the vent inlet a bit closer.