You shouldn't need to undo the nut for the bulb, just the knob. If you do remove the nut and switch, pull the negative terminal on the battery. The semi-exposed screws on the rotary switch are hot to the battery.

Don't know about the tranny/ashtray bulb. Try it and see. What's the worst that can happen - you'll burn out a bulb you don't need?

The light comes on for the rear fog, your car may still have it, assuming not federalized. Turn headlights on, then pull to the second detent check the rear of the car. If the rear light doesn't work, check that bulb before doing too much wrestling with the switch. You may need to have continuity through the rear bulb for the amber indicator bulb to work.