Yes, I think I focused on your "jumpy idle" symptom.

13mpg has to show up on the plugs, in the exhaust pipe, or in the oil pan somehow or another. If you're sure of fuel pressure regulator and ECT, there's always those less likely possibilities, including leaky injectors, blocked fuel return line, uncommon ECU issue or such. Did you cover the thermostat; is it running at normal temp? No, even an open tstat would not be 13mpg bad! What does your nose tell you about where all that fuel is going?

By the way, that strange blip at 1200 on decel might just be the ECU coming off of fuel shutdown on closed throttle deceleration, a built-in function. The fact it is so noticable might be a clue to what else is happening, but I don't know what.
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Art Benstein near Baltimore