Is Monster available in 5.1 surround sound? According to the Amazon page, the Japanese is stereo. Is this right? Is the English version have 5.1 available, like One Piece does?
No, the English version doesn't have a 5.1 Surround track. This is probably because the original Japanese was released as only Dolby Stereo and the wasn't a way to create a 5.1 mix from the source (I have no source to back up that claim), and the show is pretty long so it's unlikely there will be a rework of the audio unless there's significant interest.

I've also looked at the HK release of Monster and that is also Dolby Stereo.