Bare-Metal foil

I wouldn't try to avoid seams. BMF is so thin the seams look fine- barely noticable. I would  cut a ring, then slice an opening in the ring to allow it to go slightly cone-shaped.  Two pieces is fine too, since good seams are easy to make.

It would be tricky but you may have to try cutting it from the sheet in a way that will closely resemble the shape of the headlight when it is wrapped around the light, if that makes any sense. Minimagneto makes a good point. It might actually be easier to apply it in two pieces. It would also help the appearance if you attach it to where any seems are on the underside of the light. Either way, if you're careful about how you attach it and burnish it down real good so it lays flat and there are no wrinkles it will be difficult to see any seams. You might want to try the auto forum. There's bound to be someone over there that uses that stuff a lot on model cars and trucks that could tell you exactly how to apply it in your case.

Probably yes. I mean guys do entire airplanes with that stuff. It is likely you'll have to do it in two pieces.