1. If two separate collisions happen to occur at the exact same time (rare, but not impossible, right?) each will happen in its own separate event, right? Meaning, there won't be four instances to pick from in the event.
This is correct. Each collision fire's it's own onCollision event.
2. If three instances all collide together (I don't even know if this one is possible; just want to make sure my bases are covered) then will there be three to pick from? Or would it fire off three separate events: one for the collision between instance 0 and 1, one for 1 and 2, and one for 2 and 0?
If your picker returns 3 objects, then they will be object 0, 1 and 2 (0 based) so you would be correct.