Yeah, the fireball behaves like a regular fireball enemy in terms of both hit and jump detection from the moment it spawns, even though it starts out following a set arc. So going for it while the arc is still low is a cool idea.
You can never get points from jumping the oil can itself on the barrel stage, even if you could get high enough. That particular screen has its own routine that only checks for barrels and fireballs, unlike the other screens that just check for anything that can harm you.
