No it can not possible lag your computer as long as you have atleast 2 processors (which most computers have) as WC3 is only single threaded.
It might cause major frame skipping or even WC3 to lock up for a few seconds if done badly.
Also a decent map should not have 1000s of units as WC3 is not designed to handle that many units.
Not in theory. It might crash or show performance related lag on some systems though (due to low framerate).
You only need to add unique events, so once a unit has been attacked once there is no need to ever re add the event.
1. is it possible to detect a unit that stops attacking?
Not in theory.
Obviously as those events do not create net traffic and thus do not make lag. Running a few thousand TriggerSleepActions or damage a location calls however might as those do make net traffic.I once tested this with a few 1000 events on a rather low pc and everything worked fine
imho the problems with damage detection are overrated
I think destroying the trigger removes the events. You could also impliment a unit recycling system so the same footmen get reused.
Trigger - Add to another trigger <gen> the event (Unit - (Attacked unit) Takes damage)
a unit stops attacking after every attack
what do you want to do when the unit stops attacking?