Fist time I called a dive at depth...

[Bretagne]

voop,
I didn't answer you as I was away for some time. I agree with most of what you said (I agree that my explanation of why I need less lead at stops to be neutral when I have gone deep is not satisfactory, but it is the only one I have found).
what I slightly disagree is that:

While I write "correctly", this is one of my big(gest) issues with the French system of diving instruction: that essentially, all dives are "trust-me-dives" -- or, at least, that's the impression that it seems to leave on many divers.

It would have been trust-me-dive if I had tried to go on because the N4 didn't saw by himself that there was a problem, and things were not yet out-of-hand. The philosophy of the training is that we have to experiment several times potential narcosis depth before we are allowed to go there in autonomy. For me, it makes sense. It does not mean that we follow blindly the guide (the one who is experienced at these depth). I called the depth because I had experience enough to know that something was not normal, and I had better to do something about it before it worsened. I am not autonomous at 40 m because I do not have the experience to bring back some one from 40m, from 20m I have done it a number of times, and I continue training for it in deep pool. But nevertheless, in our training we got quite a number if informations on diving at 40m (the first one being that at this depth, do not count on problems solving by themselves).
It was my responsibility to call the dive there because I had a problem, I took it. It is quite possible that I had been able to come back safely by myself, but it was not sure, and the N4 took it it charge. It was quite obvious that I was aware of what happened, so not deeply narced (but still a little bit, and probably too much CO2 too)