Interesting thread... In almost 30 years of flying and instructing on piston twins I have not come across this "ACA" thing. Not in any aircraft manual nor in any of the training manuals of the schools I trained in and worked for. On the contrary, almost every light twin I have flown has a "ba(u)lked landing climb" table of graph in it's manual with figures for the single-engine go-around at very low level and they all start at 0ft, not some "ACA" value. Is that an UK thing or did I miss something important all those years?
And regarding the autopilot: Every AFM I know states "Autopilot: disengage" after an engine failure. Some allow the autopilot to be re-engaged after the aircraft has been configured and trimmed for single-engine flight, others don't. Although not being one of those magenta children I would agree with the examiner that an autopilot which is capable of and approved for single engine flight should be used as soon as practical to alleviate the workload.