1) It's normal that the lights dim when you engage the starter. 3-5 seconds seems long. My SDL starts within a second or two of cranking. My S420 is in the 3-5 seconds range. It seems long to me. I've never bothered to do anything about it. Maybe changing the plugs and wires (wires don't apply to your car) will allow quicker starts.

3) There's no switch. Get in front of a truck at night and there should be less glare than you'd expect from a plain mirror. Maybe a blueish tint too.

4) The Parktronic sensors are in the rear bumper. I don't think there's a switch to turn the feature on and off. The rear cabin lights have nothing to do with Parktronic.

5) Try the detailing forum for tips on removing water spots.

Sixto
95 S420
87 300SDL