You've forgotten one scenario. In my company (also allowed by FAA), we are permitted to descend below MDA with *only* approach lights as reference, but *only* if the plane is in a position to continue a stabliised descent to the runway. (with AP disengaged)
In the above scenario, you legally cannot descend below 100' above TDZE unless the runway environs are in sight. If the AP has already started to round out the descent at MDA, the airplane will no longer be "stable", and you've given up the approach.
Thanks for the response.
One minor correction: You're allowed to descend to 100 ft lower than MDA, not down to TDZE + 100. I realize that on a standard Cat I ILS these are the same, but this is in the context of a NPA.
I guess I'm not following your how descending on the lights is different. Presumably on a CANPA, you would have the lights in sight (but not the runway) at MDA + 50 or you would have initiated the Missed Approach. If you're descending below MDA, the presumably you had lights in sight 50 feet prior and have already made the decision to continue below MDA and disengaged the A/P. I'm not seeing where having MDA set in the altitude alerter would affect either of those scenarios.
And yes, I agree that descending below MDA on the lights alone is rolling the dice.