A few things.
First off, if they have your card number and haven't actually hit it up for the $2k you put down yet then you can stop that payment from ever going out through the card company as a last resort. Just incase your dealings with the dealership end up sour.
Secondly, the dealership can either get your business here or they can not get it. While this sounds pretty obvious remember that you are probably more informed than they are. There are multiple threads on this forum that state how easy it is to walk in and buy a M3 from many dealerships all over the country. I'm sure our buddy Timsev would love your business and would sell you the car at MSRP. With 200+ open allocations around the US the pressure shouldn't be on you, but on them. You should be able to get around that 10k markup when you talk to their manager. Also, if he doesn't honor your previous agreement made with the former sales associate then there is no reason that you should honor it either. Especially after they seem to have changed the rules on you.
So, I would talk to the sales manager and explain your situation. He may side with you since you've been on the list for so long. If he doesn't, then I would start pulling out the facts and the knowledge that this forum has to offer you. See if they don't change their tune. It would be a shame if they lost your business simply over a completely arbitrary and very greedy markup. I'm certain there is no shortage of BMW dealerships in socal.