Finally heard from the dealer after they've had my car for 11 + days. They told me "those parts we ordered for you are supposed to be in tomorrow". I informed the "service coordinator that I had no idea what he was talking about since no one had contacted me regarding a possible diagnosis. Yet another example of how disappointing Grappone Ford-Mazda's service department is.
Anyways, they said the gas gauge issue was diagnosed as a body control module, and (after I asked--they didn't even have all of my issues in front of them when they called) that my magically reappearing check engine light was found to be some type of a faulty vacuum valve on the vapor recovery canister that they had just replaced. Nice, reliable parts they're producing & installing!
We'll see what happens next.
Ford, you need to train your dealers' service folks better! This particular dealership has made it pretty unlikely that I'll buy another Ford.