Folks you are doing a "Phil"...I tend to over think things and worry many into place!
The small amount of water in a FWC installed hot water heater after draining, no blowing out, has never to my knowledge damaged a heater tank ...so why all the concern? I have been in -10F without any damage and with only draining the heater into the main water tank and then draining the entire system.
Removal of that exterior plug, again as I remember is a no-no for draining according to the manufacturer...why risk it if the water left in the tank does no harm?
Just me and I could be wrong....
Phil