Does anyone know offhand of any caselaw that makes a prosecutor's decision and reasoning behind prosecuting or not prosecuting an offense privileged and not subject to discovery or to becoming evidence in a civil proceeding (a civil proceeding that has nothing to do with the prosecutor)?

You might want to look at State ex rel. Curry v. Walker, 873 S.W.2d 379, 380 (Tex. 1994) (orig. proceeding). It holds that a request for a district attorney's entire prosecution file, sought in a civil proceeding, necessarily would reveal the DA's strategies and thought processes, thereby implicating the work product privilege. Its reasoning might point you in the right direction, if it doesn't directly answer your question.

Thank you very much!