That's a lot of questions, but let me answer some of them. An
incandescent light source is very efficient at converting electricity
into heat to raise the temperature of the filament. It is inefficient
at converting that heat into visible light because it creates a
thermal distribution of wavelengths that contains most of its power
outside the visible range, particularly in the infrared. The hotter
you run it the better, but if it runs too hot, the filament doesn't
last long. You can do a little better with a halogen bulb, which
prolongs the life of the filament and allows you to run a little
hotter.
It is slightly more efficient to use DC since the temperature of the
filament will always be at its highest point, but this is a small
effect since the thermal time constant of the filament is typically
long compared to half a cycle of AC. You can safely ignore the 60 Hz
radiative losses that you are worried about since these will be tiny
compared to the total power. If those losses were important, you could
always use two filaments in close proximity, with the currents flowing
in opposite directions to drastically reduce the 60 Hz radiated field.