The variable defined in the config here actually goes unused in shutdown-hosts here, and the command is simply hardcoded:
COMMAND=( ssh -o "StrictHostKeyChecking no" "$SHUTDOWN_USERNAME@$IP_PREFIX.${HOST_TO_NUMBER[$MACHINE]}" -- 'shutdown /p /f' )
Just wanted to point that out, if this was not intended. Hope this helps.
P.S. Amazing stuff!