OK, I have a buddy who agreed to try low temperature brazing - will know the results soon enough. Failing that, will do more research on KWA KZ75 and TM XDM, as suggested by Styrak and ILLusion.
Other than 'imperfect' slide to frame & outer barrel fit, is there anything else that may have caused the crack in that particular spot? Would a return spring that's too soft be able to cause this kind of damage? I mean, does the slide in recoil hits the outer barrel harder if the spring is weak?