Welcome! I live near central PA myself, and have a 67 mile commute! I'd like to think that I am in good shape to answer your questions.
Would a ScanGauge help on the interstate during cruising? YES! The difference between a TPS (throttle position sensor) value of say 14 compared to 15 is about as much as a toe-wiggle, but the difference in MPG can be as much as 8-9 MPG!
Rigid and secure wheel skirts? You'd have to build them; there's really no pre-fab market.
Mudflaps are touch-and-go; one may work better than the other. No, I don't know which would work best for you, sorry.
Airdam - While it would increase frontal area, it would make less air go under the car. That's a good thing. If you make it semi-permanent, you can try it for a couple tanks and see how it works out. If it's good, make it a permanent mod. If not, scrap it.
No idea about custom shops.
Where about in central PA are you? Feel free to add yourself to the member map!
EcoModder.com - Googlemap ME