@dc42 I have checked this and there is not short between the heater and the duet. I have also tested each individual component separately, and they all still work. Excepting the thermocouple daughterboard, we are just paying it safe and buying a new one. So still not sure exactly what happened here.
I am rebuilding with all of the tips in mind shown here. Plastic washers between the screw and board, and the board is mounted to plastic frame now. I have removed the peltier cooler from being controlled by the board altogether and am using a separate control board for this. I have docked it all in a computer case with adequate airflow and a fan to avoid dust. Hopefully all these combined will keep me good to go going forward!
Thank you to you and @NeoDue for all your help identifying points of improvement going forward. I appreciate it.