The shadow self thing is just wordplay on doppelganger. Calling it now, though, Silas was lying about how it works to Stefan. The show has already shown how nature abhors immortality, so Stefan is probably the only one who can kill Silas, or they are somehow linked and if Stefan dies so does Silas. The latter would explain why he stuck him somewhere he can't be killed for a long time.
Also, I'm betting for the season opener Cadzea does the possession thing of Bonnie's body (like Rebecca's mom did to her) and releases Stefan from the locker.