The first and easy answer: Take out the false heading from the incoming NMEA0183 stream.
Your ship symbol in OpenCPN would be turning as well I suppose?
If you have two heading sensors could you correct the bad one or shut it down?
Is it possible to filter out the bad HDM/HDT/HDG sentence from the
NMEA stream?
A copy from the NMEA Debug window or a VDR-file for a couple of minutes would help us to look further into you data if you need that.
Next.
Radar_pi: "
..the dedicated screen." I suppose you mean the PPI window. If you use only that, in
head up mode, and not "Chart overlay" you need no heading. But if you receive a heading, bad or good, into OCPN it will be used.