Total brake failure - trying to understand what happened
Since recovering from my crash, I've put a bit more thought into this.  I replaced the calipers, rotors and pads, with Avid BB5s - so very similar to the Promax ones that were essentially copies of the same design.  The Avid ones make it slightly easier to tweak the fixed pad, and this turns out to be a good thing.
Fixed pad wear makes other issues worse: One thing I've since noticed with BB5-style brakes is that they're quite sensitive to the fixed pad position, and therefore to fixed pad and rotor wear. This makes other issues worse, and unlike getting them filthy, gets worse the more you brake.  Riding in mucky conditions it's important to check and set the fixed pad clearance fairly close periodically.  This certainly wasn't the only cause of my crash, as seen from the melted material in the rear rotor, but I think it goes some way towards explaining the uneven moving pad and rotor wear at the front.  On brake with only one moving pad, the rotor is bent to contact the fixed pad.  If the fixed pad and/or rotor (in my case, certainly both) are worn excessively, the necessary bend increases, meaning two things: more of your braking effort goes into bending steel; the contact between the moving pad and the rotor is concentrated nearer the axle, producing the uneven wear in the photo above.  With slow wear this should be a small effect as the pad will take on a small taper, but if things wear too fast (or you somehow back the fixed pad off instead of driving it further in) this either won't happen fast enough, or it will wear down to the backing.
On the Promax brakes, adjusting the fixed pad with the wheel in was tricky - there was a knurled plastic part to turn by hand but the adjustment was too stiff (so I was used to thinking the adjustment had bottomed out when it hadn't), meaning using a less than ideal screw in an awkward place.  The BB5s adjust in the same way but nicer.  I'd checked and tightened earlier in the day, in daylight, but everything was wearing so fast (though I didn't know it) that I gave the fixed pads another tweak not long before the problematic descent, this time in the dark.  At the front it still wasn't enough.
Conclusions:

I'm personally not going near &quot;ceramic&quot; pads again: this bad experience; they seem to wear rotors very fast; and the description is fundamentally not to be trusted if they can melt.  For some people they might work.
Check the fixed pads frequently.  If braking seems soft, stop and tighten, perhaps also cleaning the rotor (pre-injection isopropanol swabs are good for this).
If you run out of travel, fit new pads immediately.  Wear is very dependent on riding (weather/surface) conditions as well as road surface and load, so you can run out fairly suddenly.  Range of adjustment determines when the fixed pad wears out, so it can look like there's plenty of material when the pads are no good (much more than for the moving pad). I always carry a spare set (since an earlier issue which in hindsight offered hints about pad wear) and have changed them at the roadside, mostly recently coming back from a 2-day 200 km tour with rain, mud, steep hills, and gravel (and a record 4 punctures in 2 days).