codes: these are diagnostic fault codes, stored in the car's engine management computer. your car uses electronics (injection/timing map) to figure out when, and how much diesel to squirt into the combustion chambers.
these codes usually last 50 start/stop cycles. hence, a bmw tech can interrogate the computer and figure out the problem.
2. turbo: it is spinning all the time (provided engine is runing). the amount of boost is of course dependent on the revolutions (and the consequent volume of exhaust gas. in my experience (a week or so of a 320d rental), 320d has NO turbo lag, so what you are describing can be abnormal. waste gate may also be electronically managed on your car, not sure.
this is a stick, yes?
3. search (painful, i know) for anything on 320d, cos i do remember similar problem being discussed in the e46 section.
4. i do not know anything about the emissions management of this motor, but it may be a factor. however, i think this is very unlikely: usually emission faults show up as check engine lights in the dash.