2. MY14/15 - Agree with Wildbore.
3. Red brake callipers - These are a cost option, are these on your order form? I wouldn't be happy with them being painted. Anything short of them supplying brand new red callipers from the factory is unacceptable IMO, especially as it is a £400.00 odd option, on a £90,000 car.The RS6 is still being built at MY14 specification. MY15 has not yet even been announced and - at this rate - you won't be able to even specify an MY15 car (built for delivery next year) until late this year or even the beginning of next.
4. Registering the car - While it will make little to no difference to the value of the car, and shouldn't be an issue if you're planning on keeping it. It is frustrating, were you not sent or called in to sign a confirmation of registration details? Get the dealer to confirm if the car has been registered, this seems really peculiar if you'd agreed Sept. Maybe it is a genuine mistake from admin, was your salesman/woman aware you wanted it in Sept? It wouldn't be the dealer trying to hit quota as someone suggested, it doesn't work like that.
Try and work out a solution with the dealer, callipers can be retrofitted (not painted), and the plate won't really have much bearing on future value, so, see what they can offer you and start to enjoy your car a month early.