Do attacks that hit multiple enemies (or one enemy multiple times) generate multiple on-hit triggers?
Solution 1:
For attacks that hit multiple enemies, Yes.
I haven't explicitly tested all on-hit effects. I will update this answer as I do.
Life on hit: Confirmed. With a +2 life on hit weapon equipped, I often saw healing popups of +6, +8, +10, etc while using area attacks that hit multiple targets.
Chance on hit to fear: Confirmed. Using Deadly Reach I saw two enemies get feared and run away.