Australian Car Repair Issues? Ask an Auto Mechanic for Answers ASAP
Sounds like a possible fuel pump fault or a fault with the throttle actuator. Lack of throttle response could be due to low fuel pressure. I'd suggest getting a fuel gauge connected to the fuel rail to measure the pressure. You should have approximately 52 psi when running.
It would have nothing to do with the battery replacement. If the car was fine up until the battery was replaced I'd have to go back to the throttle body initialization. Try leaving the key on for a little longer and see if it will initialize. If you have no success you will need to get the car connected to the Mercedes tester to check for faults and perform the initialization.