The screw at the rear of the headlight is to adjust the focal point and light pattern and not intended to screw tight! The bulb is not pre-focused so you need to adjust the focus against a wall.
The bulb should not touch the reflector, the bulb socket is supposed to slide in or out in the reflector socket. The Motolamp for 30-31 has a spring (original is conical) on the screw between the bulb socket adjusting arm and reflector bracket and I am not sure the bullet headlight has that.
I am only used to the ugly Bosch HerringCan headlamp on my bike. So I have to say I am not entirely certain the bulb recommendation apply to the bullet headlight and Motolamp for 30-31 They also have different part numbers in the parts book.
But I think they are same and in that case he bulbs are normal bulbs fond in mopeds, scooters, vintage snowmobiles and such light vehicles equipped with 6 Volt systems. Since the generator only puts out some 35 watts the lights have to be weak. Only 20/20 watts (or 15/15 if you use 10W rear) for the headlight and 5 or 10 Watt rear.
It is possible to use a brighter bulb (25/25W) but that might discharge the battery slowly when running with lights on for a longer time, and when the voltage drops the light gets dimmer despite the generator charging at full capacity (and getting hotter than necessary).
The headlight socket type is called BA20D in Europe and I found Tungsram has a number 1511 for the 15/15W. The location tabs is at equal heights but different widths in the Herring Can.
The taillight socket is called BA15S and Tungsram number 2639 for 10 Watt.
I don't have the instrument light at hand but I think it is the same socket as the tail light BA15S but it is 5W and that has Tungsram number 2615. The light cover gets awful hot so if you can find a 3W light or LED light for that, it is better.
US have a different and (to me strange) bulb numbering system.
Now when you are running all lights on, the generator is running at max capacity and very little or no current is running to the battery. The Amperemeter is flickering around zero when keeping the revs up on the engine and turning to discharge when the revs drop. With all the lights off and when the battery is fully charged, to compensate for internal losses the amperemeter should be reading some 1-2 Amps with a smaller battery and maybe 3-4 Amps charging with a larger battery capacity.
With all the lights on, the remaining 5 watts that the generator is capable of charging is easily disappearing in the resistance at connections and harness wires. So you need to keep the electrical system in very good order, use heavy gauge wires and keep the connections tight and clean (soldered).
It is best to fit as a 'large' or high capacity battery as possible that fits in the battery box in order to have as big discharge buffer as possible. Original is 16 AH and the miniscule 6V batteries found in some small vehicles will strain the charging system more than with a bigger battery because the constant discharge/charge going on with a small battery.
But most important is the generator regulator. In pair with poor connections I feel that is the main source for electrical problems and the archaic cut-out system with manual adjustment of the third brush on the DU5 doesn't suit modern more sensitive batteries that has thinner lead plates internally, is just inadequate and should be tossed on the heap for a electronic regulator and easy reconstruction to a 2 brush system. I recommend that to save a lot of headaches.
Your assumption is right, the magneto supplies the power for spark and the battery is not used at all for ignition. But normally the engine is killed with the exhaust valve lifter, either rod through the tank or the lever behind the oil pump. The kill button on the handlebar is a nice option. Standard on all Indian Fours and I think standard on sport bars.