1. turner motorsport chips are excellent chips. i have had excellent results with one in a 325e as well as a 325i.
2. don't know, but there isn't a lot of extra room in the ecu, i suppose you would have to put the 2nd chip outside of the ecu.
3. if available, otherwise use the highest octane available in your area. you could get pinging and detonation using low octane.
6. to the best of my knowledge, ac schnitzer has never made a chip. with ebay chips you can't be sure what you are getting or who did the programming. i wouldn't recommend them.