Could you explain how you are going to use the 30 Amp Ford Mobile with a 20 Amp 240V receptacle?
there are many, many 240v adapters out there...
we don't know, yet, whether there is a derating of the amperage with the Mobile Charger, or within the Lightning, to limit the 240v requirements below 30amps, down to 20, or 15, etc. We DO know that the higher cost Charger(48amps) will have that option.
as for the '80% rule', that has nothing to do with the outlet or the wiring or the EVSE, only with breakers, if it's something that concerns you. Personally, if I have a 30amp breaker, I suggest that it's quite capable up to 30amps, and the same for a 50amp breaker.