I can't help with experience with the FEH, as mine is not yet produced. However I can advise that it may take 3,000 miles of break-in (loosen everything up) before you begin to see the long term fuel economy numbers that you expect.
Some posters have said it continues to improve up to 5k or so.
I wonder if the gas engine is kicking in at 10
mph because the battery is not fully charged. Perhaps a road trip is in order.
The Ford technicians are correct in stating that it is likely normal experience. I'm sure some of the experienced pros here on the board can give you advice on improving your MPG.