I would not say the shop is dishonest. Although they should not be just throwing $1500 parts at the car. They did find the error with the air injector system. They just jumped to conclusion it was the pump. I would say they are not familiar with the car. You should look for a independent shop that works on German cars.
Google Crossfire RCM and you will find videos of the replacement. Very simple swap out. Even if you do not do it yourself you can show your mechanic so he can do it.
The members telling you it is the RCM are very knowledgeable.