Microbus,
Correct,the PTU shuts off when the alt flaps switch is moved to arm position and TE flaps switch selected to the down position.
Correct,you cant retract the gear if you lose system A,the Landing gear transfer unit works when Eng 1 fails,therefore its main pump as well.The remaining electrical pump would not be sufficient to retract the gear in the time limit imposed by certification(8 secs i think)so extra sys B pressure is given to retract it.