There is an understanding that if you're a good FFESSM diver, you'll make a poor military diver.
I'm pretty sure it's the opposite. FFESSM was founded by military divers, but they opened diving to women in the 1970s and relaxed many physical requirements over time.
N2 is limited to 20m without a DM and 40m with a DM. GIven that the overwhelming majority of rec divers dive with a DM, that makes sense.
Of course an instructor for PADI who never did a deco dive in his life would be limited to either 20m or 40m within the NDL.
You are right, the reason FFESSM cert. is mandatory in France is because the FEESSM writes the law on the practice of diving. However, the FFESSM introduced the "aptitudes" or capacities which allow someone with no FFESSM/CMAS cert to legally dive in France. These capacities are only granted at the discretion of the dive director, though. It's merely a question of legal liability.
If people can't manage narcosis at a given depth, they shouldn't dive to said depth.
The Trimix cert allows diving to 70m, which follows an incremental logic from the 60m on air allowed by the N3 cert.