Good luck! You're welcome. I don't think anyone will walk out of your lecture after the bait-and-switch. Maybe you can hedge the bait-and-switch by making the main topic 20% of the total time and the rest 80% (Q&A + the switched topic). Please try to video or at least audio record it for your records (you seem to be someone who will want to do this sort of thing again, better). If you can get pizza (even if its - who wants pizza - pitch in a $1 or $2), that might help.
