Your speculations are all great but unfortunately I highly doubt that Peter was a playable character in Verrücktainly becaus of two things:
1.) Peter was killed by Edward Richtoften after peter discovered that Richtoften was an Illuminate
2.) Samantha never knew Peter until Richtoften found him in Shi No Numa and hanged him, but placed the parachute near him to make it look like an accident the blank picture is probably of her father Ludwig Maxis.
Sorry to rain on your speculation but there has been a shown storyline and the radio messages to Peter when you knife them in the game are probably those of Dempsey or someone else trying to find Peter because when Peter heads to Shi No Numa Dempsey, Nikolai, and Takeo arrive at Der Reise.