I feel that there is not enough information to make a judgement. A shop has no right to do anything without authorization. Who was doing the authorizing? It sounds as though there could have been communications problems that kept the shop from pursuing more than had been authorized.
As was said, you do not know how the car was before the servicing. The description of the service indicates to me a fuel system flush and or throttle body cleaning was done. I don't see any particular reason to assume the problems you described would be involved in such repairs. Maybe if they cleaned the throttle body.
Since it sounds like only one side is left to tell the story, you should assume that there were reasons for years of trusted service. Ask them to go over the repairs with you. If they are not the shop for you. It isn't worth arguing over.
Bosch Master, ASE Master, L1
33 years MB technician