Welcome to the site !!
It seems to be a issue with the lighting kit from your description , as the lighting coil and charging coil and ignition coil are all independent of each other , so running without the light should not affect the ignition side , Baja guys do it all the time , unless its backfeeding somewhere in the system , and draining amps from your ignition side of the stator , i would think that something in that kit has possibly already damaged something and this is why your getting normal test results when the bike is sitting , but under a load the system is breaking down , though it is possible that running it with the short could have already damaged either the stator or CDI or even coil , so that one of them is breaking down when the load on the system is changed by removing the bulb , like as if the light was completing the circuit or something , to me the culpret is the kit , the issue is , what damage did it do if its shorting out or backfeeding the system