Ian,
Those quotes were not mine, but to take up a couple of your points:
The crew were visual with the runway (report on the CVR).
I'm not so sure. I don't understand why one of the pilots could be visual with the runway but fail to see the 4 reds of the PAPI, which should then illicit a startled response along the lines of "@#$% we're low!". A possibility is as mentioned previously that the caller called a road or other lights, not the runway.
What are the 'published minimums' once visual with the runway?
There are none. It's a Visual Approach from the point you get Visual. A distance/altitude scale would be of limited value at that point because eyes would be outside, on the PAPI.
The NPA had 'done its job' and the crew were now runway in sight landing visually.
Pure conjecture, but I don't think it did. It did not leave them at the MDA on the 2 whites and 2 reds of the PAPI. If one gets Visual beforehand eg off a Dive and Drive, you'll see 4 reds; I would then hold the altitude and fly into the PAPI 2W/2R and recommence descent.
Another concern is the "Sink Rate" call. The autopilot was still engaged; if in Vertical Speed set by the PF (I'm not familiar with Airbus), that would be a rate of descent of at least 1000ft/min (the warning in my aeroplane goes off at around 1700ft/min at 500ft AGL). The terrain was undulating, which can trick the system, but nevertheless the rate of descent would seem to be on the high side to me.
unforgivingly small vertical margin for visual approaches.
The excellent (unverified) profile diagram of MM43's shows that to be the case, in my opinion. By day, you'd see the ground close-by and subconsciously not go low. By night, it'd be invisible (apart from a red light mentioned earlier?) and if the crew got low for whatever reason they would have few visual cues about how dire the situation was.