B&M lights and associated gadgets are distributed in the USA by Peter White Cycles. He sells them direct, and he keeps some retailers going as well.

Soubitez, Union, and a bunch of other vintage bottle dyno companies were bought by Marwi in Germany, and live on under that flag. Other assimilated brands include AXA, Basta, and (I think) Spanninga.

I've found a lot of those vintage dynos still work quite well. A lot of the drag can be mechanical, based on too-tight bearings with grease that is congealed and gooey. Cleaned up they can become really nice. Don't damage the actual wires when you have it taken apart!

When you install and wire up a system, there's no reason not to use a return wire to close the circuit back to teh generator, as long as you can electrically attach the wire to the generator body and to the actual negative connection of the light. This is actually a ground loop but there's really no way interference can result if it's just a dyno and a headlight, so electrically it's ok to do this.

For a carbon frame, yes, carbon fiber is conductive, but it's a much worse conductor than steel is. Problem is that fiber structures for bike frames are not designed for conductivity, so if the current loop is actually closed it might not stay closed. So I think wired grounds are necessary with a carbon frame.