I can not directly answer your questions but have some comments. That aside, the LED usually blinks at the speed of the zapper. The human eye can not detect a flicker faster than about 16 HZ which would mean that the 15 Hz zapper would have a visible flicker but higher frequency zappers would not.
Returns: Most zappers offer at least a 30 day money back guarantee.
Smallest zapper: According to our studies, the smaller the contact area, the less effective the results.
For more information, read " parasite Zapping and The Zapper".