Sakura-san didn't seem heavy to me. Maybe someone of average strength would be able to carry her.

[But the piano is an entirely different matter]

It isn't easy to break a piano, so no idea about that part. If someone grabbed Sakura-san and smashed her against the piano I think that would indeed require some strength to break it...and her body would be much more broken than it was.

We could suppose it wasn't broken because she was slammed into it.

[Or maybe it was a pretty weak piano.

Mouri has gotten behind Black. With subtlety, he opens his handkerchief to compare Black's hair against the ones found in Megumi's fist]