Can an employee grieve an executed SRPE, regardless of venue warranting the SRPE (i.e. during annual mandate, during CSL, etc.)

Yes, bargaining unit employees can grieve any personnel matter if not specifically excluded from the negotiated grievance procedure and this doesn’t appear to fit any of the subject matter exclusions IAW 5 USC 7121(c). Non-union employees may also grieve under the administrative grievance procedure.