i dont believe there is a perfect way to show a JW the stupidity of the believe system.
yours is nice, but it needs a JW who actually KNOWS what happened more than 5 years ago. i made the experience that at least 80% of all JWs dont know shit about their religions history. they're repeating some crap in the book study to answer questions, but they dont LEARN it. the 1918/1919 date is unknown for many many JWs and most would say it were the witnesses who were selected then, not bible students.
PLUS:
the discrepancy between the back then bible students and todays witnesses is that the witnesses STAYED with gods faithful and discreet slave while the ppl who call themselves bible students today are a sect who didnt want the changes god did to his organization. the modern bible students are the descendants of *apostates* from that time. if new light comes from the governing body today and someone chooses not to accept it hes apostate. and if he chooses to create a new religion by staying with the old light hes even worse! the "new light" doctrine can twist things this way and every JW will accept it. not a big deal in my opinion.