I am not going to trade in a 3 year old truck (with 13500 miles!) on a new one unless there was something that was MUCH better. The 6G isn't much better, much less MUCH better, so it would have been off the table anyway. The fact that there is no Super Cab or regular cab put the nail in the...
Yup, "cattywompus" is a real word. I know it is because my dad was using it way back in the 50s, if not earlier. I don't remember anything before about 1956 (when I was 4).
Given how much effort Ford (and all the other mfgs) put into making their CAFE numbers, I doubt that their are any engine mods to be done to increase MPG. Those things you mentioned are to add power, and that will decrease your MPG. A hard bed cover (tonneau cover) might help a little.
There are other threads on this subject. It seems to be a symtom of a failing battery.
I understand the mileage thing. I am also retired and just rolled over 13K on my 2020 XLT. When I was (much) younger, I would do that many in about 6 months!
"Our 250cc engine makes 16HP". My 1966 Honda 305cc motorcycle was rated at 28 HP. That doesn't sound like "twice the power" to me.
I also don't see a way to scale it up to multiple cyclinders, unless I'm missing something. That is very possible.
I think there is usually a board embedded in the fiberglass over the bed rails. You could just put a couple small screws through the shelf into the fiberglass and call it good.