Hello
Well corrosion at the CEM and connector is well documented for P3 XC70's of that era due to poorly applied windscreen sealant allowing water in....so you could at least check the plug connector on the CEM for corrosion and water ingress by checking under carpets to see if they are soggy underneath.....you have nothing to lose from cleaning and re-seating the connector and see what gives.
You have already noted that there is a brake pedal position sensor issue and it could quite possibly be as simple as a faulty pedal switch.
HTH
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