2)Xenon's are direct driven just like the old incan torches.
3) No reason you couldn't if they fit.
4) an 18650 is only 4.2v tops. With a Xenon the more voltage the hotter the light and better chance to start fires. This is why one would use 2x16340 (8.4v) instead of a single 18650 (4.2v) in that UF-50x host.
I would add a 27mm aspheric lens to that puppy to focus the light even more. Pin point heat from a few inches. Use it to light your cigar. Impress your friends. Create fear in your enemies.
The 28mm lens from DX is really can’t fit into the head and it is about 1.5mm+/- bigger. I took out the original lens and measure it with a ruler (since I don’t have the tool) and found that WF-501B’s lens is only 26mm+/- and the lens from DX is 28mm! Unless you have a very good grinding skill (grind away about 1.5mm+/- from every side of the lens ) , if not the lens is useless. May be this is why he use C1 to make the burner.
I have been searching for the method to make this burner over a month. What I found is, 9V bulb drain the batteries extremely fast, so imagine if you using 14V, will it be faster (lights up a paper and batteries depleted)?