How much memory pressure will BWAPI put on starcraft process if it keeps Unit objects allocated for every unit that was ever created during the match?

For how long will pointers to units be valid, after the unit has been destroyed?

Is there a way to detect if unit is being attacked, with what damage type and which unit is attacking it?
I can check if hitpoints have decreased compared to the previous frame, but is there a better way?

Two BWAPI questions:

1) How can I disable Broodwar's pathfinding in BWAPI?
I want to order unit to just go in straight line to the destination and stop in case it hits an obstacle.

2) How can prevent my units from auto attacking enemy units?