It sounds like an electrical problem to me, as the gearbox is electronically controlled. Reverse in L gear, that must mean that the gearbox is getting a signal to go into R. At this point, seeing that you dismantled the shifter, you should consult Volvospeed.com or the archives for alignment of the PNP switch. I have never done it myself, but I read that you can do it simply by calibrating the switch with the shifter in R and centering it using the reverse lights as a guide (engine off, of course). The PNP switch needs to be aligned properly so that the shifter positions relate to the corresponding positions on the PNP switch, and the PNP delivers the correct output signal for the position of the shift lever. I would definitely try that first. Gearbox failures at that mileage are pretty rare. Best of luck.
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'94 854 N/A.