Didn't I also read somewhere that the piano on Leaving This Town was overdubbed in the studio due to some technical problem? Therefore, it may not be Carl who played it even though he might have done so live.
I would tend to think it was Carl, even if it had to be overdubbed. From what I've read, Carl and Ricky were the two Beach Boys involved in the mixdown. Carl was there every day, so I don't see why it wouldn't be him, since he already knew the part.
Thanks! Always loved the sound of that piano. I don't know, it has an almost sinister feel on this song.