Said: "If you look at the grille (and above it) that brings the heated air into a gas dryer in the drum..it is often stained around that area (brownish not blue/green)."
I just looked at my Neptune dryer... been in frequent use for the past four years. Not even a hint of discoloration inside the drum by the vent. This dryer has a bright white enameled drum so it would be visible if there. I also checked the drum vent in my older White/Westinghouse gas dryer. It was in regular use for 10 years. No sign of discoloration there, either.
Said: "but there is also (among others)........Nitrous oxides (NOx), Sulfuric oxides (SOx) etc."
In a properly tuned appliance, 99.9% of the fuel is converted to C02. nitrogen oxides (N2O, not NO2) would be insignificant. They can be an issue when oxygen content is limited, as in the case of high efficiency boilers and furnaces, but typically this is not an issue for appliances such as clothes dryers or ranges.
Sulfur oxides resulting from combustion of pipeline quality natural gas are low because of the limited sulfur content of the fuel. If your electricity is generated by plants using petroleum products or coal, then the sulfur and nitrogen oxide emissions for each BTU you consume are about 1,000 times higher than by using natural gas directly in your home.
My guess is that most, if not all, of the discoloration that might be found around a gas oven is from broiling or roasting meat products. Any smoking during these processes is from incomplete combustion - but not of the natural gas. It's incomplete combusion of the fats, oils, and other substances found in the food being heated. I don't want to minimize the hazards of incomplete combustion of natural gas, and there is a good reason why furnaces and water heaters are mandated to have exhaust flues, but I think you are exaggerating the problem for cooking appliances, and misinterpreting your evidence.
