It must have something of a delay because I recall our priests playing with the understudies making them tank each other after the fight - they didn't seem to die instantaneously. Also, looking back through my combat log, hits were landing for normal looking damage until this one and the understudy keeled over.
Only the understudies that aren't currently mind controlled when instructor dies seem to get the weakness increase. Those are usually killed by the warrior tanking them so no time to play around anyway.
I wonder how feasible it would be to pull patchwerk to the understudies... haha.
No purpose since the debuff is on the understudy. Meaning the understudy takes 5000% more dmg, he doesn't deal it. In this case one of the understudies hit the other while the other was affected by the debuff.