I think you made some good points especially regarding total lifecycle emission and environmental impacts. Ammonia production is also fraught with hazards.

I do have an issue with your chemical equation. It is not balanced and if the reaction temperature is moderated then NO2 creation might be non-existent. I would expect that a Ammonia air fuel cell could be developed that produces no oxides of nitrogen. Oxides of nitrogen are also a useful product if they could be produced and captured. That would be something that could be done in a powerplant. If the process goes as you projected oxides of nitrogen dissolved in water could form nitric acid which can be used to make fertilizer or explosives.

Any way the balanced chemical equation could be 2NH3 + 2O2 → 3H2O + 2N or it could be 4NH3 + 8O2 → 4HNO3 + 4H2O or if the reaction proceeds as you suggest 4NH3 + 7O2 → 6H2O + 4NO2. None of this deals with the reality that we would be using air and not pure oxygen that would also include more nitrogen and water as well argon and CO2. It would be interesting to see the design of a power plant to use ammonia and air looks like.

TEK