I would only add in one additional item to the demo and keep it simple and covert. After the householder asks about mistaken dates or something, you could say, "you must be referring to what happened in 1975 or in the 1980's when the Watchtower said those who saw 1914 would not all pass away?" Householder says yes, and then you move on.
Better yet, then have an accomplice kill the mains electricity, so the main hall goes to pitch-black until the backup batteries turn on; then afterwards, call it a miracle, a sign from Jehovah. :)
NS said-
I would start out with: "I've been asked to give a talk on if Jw's are false prophets.....But the question that totally burns in my soul ........ Are Jehovah witness a dangerous cult. And the answer is Yes.
Well, it's amazingly weak on the supportive-evidence front to support that conclusion (and it's not like there's not TONS of evidence out there), but at least the conclusion IS correct. It's just not anywhere close to resembling a fully-formed argument, and thus has a squat chance of being effective except to convince the incredibly gullible and weak-minded, who'd have to take it on faith, your word (the very ones who are MOST likely to STAY IN the JWs, due to social pressures; they trust the power of the group more than individuals, esp those speaking apostate-talk without bothering to explain WHY they believe what they do).
Adam