Wow... evidently the [edit] tags actually do something here? I tried to edit this in last night but it wasn't showing up now that I looked at it today.

Nope, the socket is just molded plastic and two copper contacts. It's possible the resistor is shown there because the bulb exhibits some unusual electrical characteristics compared to other indicator bulbs... I've seen LEDs in other circuit diagrams shown as a normal bulb symbol with a diode in series.

Well, I have a theory regarding the errant average FE readings: The control box's internal battery. I took a trip to see my brother in San Francisco over Labor Day weekend, and returned to find the average MPG displayed had dropped from the mid 20s to 4.2 MPG. I think that the ~20 year old battery must be depleted, or have very little charge capacity left, meaning that if you don't power up the computer on a more or less daily basis, the stored info gets corrupted.