Generally, retention of the safety is accomplished by the extension from the Sear Spring onto a notch in the shaft of the Safety. Somehow, the Sear Spring extension is not matching up with that notch, or the notch is not present. I would replace the Safety if that is the case. Also, you could have the wrong Sear Cage for that Safety, that does not allow the Sear Spring extension to align with the Safety's Notch, if present.
As for the Magazines, I would be more inclined to try a standard Mec-Gar Cz75B 16 round Magazine and see if that would be a better fit.
Otherwise, I can't say.