Am I in the right area to conclude that the regulator and the ammeter are both probably faulty?
Not sure about the regulator, but it seems your ammeter needle is stuck.
As you are on 12V I suppose it has electronic ignition? Electronic ignition takes a large chunk out of the power budget. If I do a quick calculation only for the lights, most tail/stop lights are 8/20W, add to that 10W from the pilot bulb (I run it on pilot) + 5W speedo light makes 23W and up to 43W when you hit the brakes. All this sure makes the dynamo work hard for the money
I have to charge my battery from time to time (about once every 6-8 weeks). When it's fully charged at around 13V, after each ride with lights on I seem to have lost about 0.10V. At 11.5 volts it stabilises and stays at 11.5V for about 3-4 weeks, and then it starts to drop to 10.5V. By that time I can see the lights go dim at low revs, and I know then it's about time to hook her up to the charger. If I let it drop any lower, the ignition starts to advance to much, and she starts to backfire.
I am considering LED tail and stop lights to have a bit less consumption.