But you were reading on the stator side weren't you? So, excess voltage in the area of 18 volts going to the battery is indicative that the regulator isn't limiting the voltage to the in-spec value, far-below your 18 volts. Since the regulator-rectifier is two electrical components on a common chassis, one limiting voltage, and one converting AC to DC, the voltage regulating function is obviously faulty.
I remember learning about the R/R function on my Yamaha 360 two-stroke dirtbike when the regulator went out, and the headlight filament popped after becoming 'really-bright' for about four seconds. For a moment, I was thinking, "wow, I'll never complain about night headlight function again!" Then the headlight blew. This was back when bikes had headlight on-off switches still, and two-strokes punished the four-strokes daily on the roads, the trails, and the race courses.
SOunds like your troubleshooting will pay-off once the new MOSFET R/R is installed.