1.what the mech may be talking about is that you have clogged catalitic converters. but if this was true you would have a check engine light on, so i would say leave it alone, don't spend $500 on something that you don't even know if something is wrong with it. if anything take it to a bmw certified mech and get another opinion.
2. don't know
3. you can replace the light behind the display, don't think it is an led tho
4. dealership can order you a new key, i think it cost me like $30 but i really don't remember.... maybe jsheares is right
5. never head of fog lights being a common problem
Mods: Tint, Euro Tails, Smoked corners and Side Markers, E46 Style Shadow Grill, 18" M3 Replicas, M3 Exhaust, Low Beam Silver Stars