...Any how, my question: Is it possible the mechanic at Certigard has done something such that antifreez radiator coolant is now going into combustion chamber...causing this code to come up ??....
No, to do this job he never touches the radiator or the engine, its the transmission and the combination trans cooler/AC Condenser. For it to be even possible, he would be taking apart things he would never have had to touch to result in what you're asking, so no, you're NOT getting fluids into the intake and thus exhaust of the motor, you can safely rule that out.
He would have to bang the exhaust pretty darn hard to crack/tear it open, and you would be getting a different code for exhaust leaks, and it would be one of the upstream O2 sensors, NOT the downstreams like the code indicates.
BUT, code P0137 is low voltage on the 2nd O2 Sensor on Bank1. The lines are difficult to route in and out of the crossmember, and you could end up forcing them a bit and twisting and turning them to get them in or pull the old ones out. The wiring for the O2 sensors, especially the 2nd O2 sensors could be in that area, and the mechanic might have damaged it.
Or it could be a coincidence and the O2 sensor just happened to fail shortly after having the job done.