If the prisoner has to be from Slytherin, then my money's on Snape. I just can't figure out how Snape would know Peter is a rat. Also, why would Peter have remained a rat when Voldemort came to power? He would have been on the winning side then.

As for the time turner, maybe Pansy? Or Hydra, so she can take everything her mother wants and still take what she wants as well?

Yeah, no idea.