I seem to recall the the ADP is automatically signalled to run when the take off roll is commenced. Whilst it is a "demand pump", the MEL limitations are huge if it is inop. There is a penalty of about 18000 kgs if the ADP is inop, whereas the number two electric is allowed with no penalty if inop. No 1 electric is not allowed U/S (reserve brakes & steering). Basically, the ADP provides the muscle for gear and flap retraction. It needs engine bleed air for power, hence no supplementary procedure for "bleeds off". Packs off is OK, commonly used.
The 757 is a different bit of kit. You can do a bleeds off as there is no ADP.
As for high altitude ops, LBA is very challenging.
