For various reasons I haven't been looking here much lately so have come in at the end of the above chatter. I gather from what has been said on previous occasions that you are writing about your 9D powered Waratah/Excelsior R. If so you don't seem to have a relevant Villiers manual which does make it a little tricky. I'll start with the first question and try to work through them all.
Of course there's no voltage control. Can you think of any British bike of the time that DID have voltage control on an AC system? I think Triumph were more or less the first major factory to fit alternators. They had the same lack of voltage control and those following weren't much better. I remember much unhappiness about batteries boiling dry and other woes in the early years of it all and a distinct lack of enthusiasm about the infamous multi-terminal Lucas combined ignition and lighting switch. What was it? the PS or PSR8? It's no consolation but Villiers were no more and no less ghastly than much more expensive machinery. Let's face it post-war British motorcycle electrics were never world leaders.
Essentially the factories fitted only the one size 6Volt battery, I don't think they had much option. I think it was the only one available and it seems as it acted as some sort of electrical buffer in these systems. They all seemed to revert to the old three brush dynamo idea of a daylight charging rate and a night time head lights on charging rate which never worked particularly well. I'm no electrician, maybe there was some sort of secret to it.
Next, there were two lighting systems used on the 9D. From memory, 18 watt or 24 watt systems. The lower wattage having both fewer coils and smaller magnets. You appear to have the lower wattage cheaper system fitted. Perhaps this was one of the ways the factory differentiated between an Excelsior or a Waratah. I don't know. Whatever, they are both standard Villiers systems of their day but you have to work to the limitations of each.
As for light switches Villiers supplied a relevant light switch for whatever lighting system you used, rectified lighting with a battery or directly off the lighting coils. The four position switch was late on the scene, well after your wee bike. Before Villiers provided this switch to change between steady battery lighting for town use and 'direct' lighting for country running the knowing rider wired in an extra dip switch and used that to flick between the two systems. Eventually it dawned on Villiers that they could add the dip- switch trick to their standard switch and sell it as a standard function but as your bike predates this you could use the extra dip switch concept as being absolutely correct for the time. Just remember that LED's probably wouldn't like the change to direct lighting too much. They're rather keen on a steady current without erratic swings. It's an arrangement for old fashioned bulbs only.
Finally I'm in complete agreement with the idea of a brake/stop light on any bike. I have absolutely no faith in any following traffic.
Anything else I need to cover?
