your gearbox is basical;ly a manual box and as far as I know no differences accept that the ACS actuator does the job of the clutch on a fully manual version , I thiunk if you had actuator problems you would get an 'F' display so I would discount that one, you could check the sump breather tube that connecting the the ECU induction line and MAY have some bearing on the tick-over which could be causing the problem , but I've not hjeard of thios problem before.
There is information on my page 59b that will explain the system every little helps.
I can't think why you would want to pree the accelorator and keep your foot on the foot brake, normally the car is help on the hand brake, see also the instructions for driving ACS which I have taken direct from the owners manual, which makes mention of holding the car with the revs.
Bert Rowe's-Mercedes-Benz 'A'-class info. Mercedes- Benz 'A' Class (ACS) Auto clutch-less transmission