Could an OPV problem affect boiler fill levels and might that be the source of my grouphead temperature fluctuation?
No, OPV and boiler fill are unrelated, sorta.
HOWEVER, it certainly appears as though you have a leaking or intermittently leaking OPV and that would, in fact, flow water through the hx and out the OPV back to the reservoir even during a boiler fill - see schematic below.
The now cooler hx would remove some heat from the group and lower its temp. So, time to remove the OPV. But before you do, order some replacement parts from CC (new OPV "rubber" insert, new o-ring, food-safe grease).