Yes, they can change what they want, but there's not much point in singling out Mystical Fire in that case.
The move description says nothing about rolling, though.
Hail is slightly ball-shaped, but shards are an entirely different shape.
Good will. What about Heavy Slam? Or other moves that should or could be counted as ball, blast, cannon moves? I know Heat Crash/stamp is out of the race but some other fire moves should be there.