Does the Soldier's Black Box heal multiple times for +15 a piece if it hits multiple people, or is it only +15 if it hits anybody at all?
Can this effect crit? (can any healing crit in TF2 or have I played WoW too long...)
Yes, you heal more if you hit multiple people. I just tested this. However the GUI displays the +15 over each other.
If you want to try it yourself, I can recommand using tr_walkway. Just set constant resupply off and bot movement speed to zero.
Yes. The more people caught in the splash damage, the better.
The healing effect can't crit, but will heal the full amount even if you barely deal damage to them.