When I had my odometer gear replaced, I asked the technician/mechanic if I caused the tooth to break by pushing on the reset button while the car was in motion. He told me that the gear is released when the button is pushed and would not be the cause of the breakage. Apparently he fixed a lot of Volvo 850 odometer gears and the OEM gear is quite pliable. He showed me the broken gear and it was just one tooth broken off.

As for the ECU keeping track of the mileage, it's probably true for the later models but apparently not true for the 93-94s, because the tech told me there was no way for him to retrieve the mileage on my model. My car doesn't have the OBD II and there's no scan tool for my model, just the plug in the engine compartment that flashes the codes. Cool, I got a few thousand extra "free" miles out of the car.