N20 formation route is third body reaction that depend on pressure. It is reported by Mellor and co-worker that N2O formation route becomes comparable with the other two routes (See link https://www.jstor.org/stable/44746494?seq=1#page_scan_tab_contents)

To answer your second question if we increase the pressure the Nox formation by N20 route will increase (Reason main reaction of Nox formation by N2O route is N2O+ O ----------2NO which is bimolecular reaction independence of pressure but N2O increase lead the reaction in forward direction) See Link :Chapter On Board NOx Prediction in Diesel Engines: A Physical Approach

To answer your third question. Krishna et.al reported that "catalysts used in SCR after treatment system are also shown to decompose and/or reduce N₂O to N₂ under select condition" (See link :-https://www.jstor.org/stable/26278393?seq=1#page_scan_tab_contents)

Diesel engines are one of the largest contributors to environmental pollution caused by exhaust emissions. The four main pollutant emissions from diesel engines are carbon monoxide-CO, hydrocarbons-HC, particulate matter-PM and nitrogen oxides-NOx. For perect thermodynamic equilibrium, the complete combustion of diesel fuel generates only CO2 and H2O. Bur for many reasons as: the air– fuel ratio, ignition timing, turbulence in the combustion chamber, combustion form, air–fuel concentration, combustion temperature, etc. a number of harmful products are also generated during combustion. Usually, in the diesel engine combustion process, the nitrogen in the air does not react with oxygen in the combustion chamber and it is emitted identically out of the engine. However, high temperatures above 1,600 oC in the cylinders cause the nitrogen to react with oxygen and generate NOx emissions. So the major influences of the formation of NOx are the temperature and concentration of oxygen in the combustion. The amount of produced NOx is a function of the maximum temperature in the cylinder, oxygen concentrations, and residence time. Most of the emitted NOx is formed early in the combustion process, when the piston is still near the top of its stroke. This is when the flame temperature is the highest. Increasing the temperature of combustion increases the amount of NOx as much as triple for almost every 100 oC increase. NOx are referred as nitrogen oxide (NO) and nitrogen dioxide (NO2). NO constitutes 85–95 % of NOx. It is progressively converted to NO2 in atmospheric air. NO is a colorless and odorless gas, while NO2 is a reddish brown gas with pungent odor. Road transport is the most important cause of urban NOx emissions worldwide contributing to more than 50 % of the NOx. Engine out NOx levels are lower for diesel engines than their stoichiometric gasoline counterparts reflecting the somewhat lower combustion temperatures in the diesel engine. However, tailpipe-out NOx emissions are higher from diesels reflecting difficulties in reducing NOx to N2 in the highly oxidizing environment of diesel exhaust, which typically contains approximately 8% O2.

So, diesel engines are responsible for about 85 % of all the NOx emissions from movable sources, primarily in the form of NO. NO and NO2 are considered as toxic; but NO2 has a level of toxicity five times greater than that of NO and it is also a direct concern of human lung disease. NO2 can irritate the lungs and lower resistance to respiratory infection. NOx emissions contribute to acidification, formation of ozone, nutrient enrichment, and smog formation, which have become considerable problems in most major cities worldwide. In the atmospheric air, NOx emissions react chemically with other pollutants to form tropospheric ozone (the primary component of photochemical smog) and other toxic pollutants. According to Euro standards (I to VI), NOx ranged between 8 to 0.4 g/kW hr. So far, exhaust gas recirculation,EGR, (exhaust gas is recirculated back into the combustion chamber and mixed with fresh air at intake stroke), lean NOx trap, LNT, (NOx-storage-reduction (NSR) or NOx absorber catalyst (NAC)), diesel oxidation catalyst (DOC), and SCR catalysts (SCR is used to minimize NOx emissions in the exhaust gas to utilize ammonia (NH3) as the reductant. Water and N2 are released as a result of catalytically conversion of NOx in the exhaust gas. Due to the toxic effects of NH3 and to prevent burning of NH3 in the warm atmosphere before the reaction, NH3 is provided in the from of an aqueous solution of urea, or DOC with SCR. DOCs may also be used in conjunction with SCR catalysts to oxidize NO into NO2 and increase the NO2 ,are the most focused technologies to substantially eliminate the NOx emissions. As EGR technology, LNT technologies are imperfect to provide desired NOx emissions reduction. You can possible meet the current emissions standards by using SCR technology.

Thanks to Dr Syed Mughees Ali for his good way of representation of N2O formation and I like to appreciate him. Along with this, the explanations for the formation of NOX by different conditions explained by Dr Saad El-Sayed are also praise worthy.

one thing is sure, contribution of ambient nitrogen is definite in formation of NO2 under high temperature conditions. So if do you exclude nitrogen from the intakes air you can have a guess on how to make this. Exclusion of nitrogen is not that difficult by the use of a molecular sieve membrane

Nice Dear Saad El-Sayed