Well, never mind on the hitch/bike rack inquiry... my son and daughter in law have given it more thought, as well as going out a couple more times with their regular 2-wheeled bikes, and given the extra challenges and expense of not only a trike but also more elaborate bike rack and hitch, they have decided to just stick with the regular bikes (they think that my DIL will be fine enough with it since her black out episodes are pretty rare and usually give her a bit of warning) .
Thanks to everyone for the wonderful advice when I was trying to figure out trike options. :-)