I am no fan of "satnav idiots" either and will do what I can to help, but within OSM's rules.

Some of these problems are very difficult or impossible to solve, unless Cornwall CC bring in additional restrictions with traffic orders.

There is no street side imagery of the affected roads available with an OSM-compatible licence. If you or other local residents are able to capture and upload imagery with the Mapillary app, that could be very helpful.

For narrow roads, you can use narrow=yes and lane_markings=no. If possible, add est_width=*, or better still width=* (but this requires using a tape measure) with a single value at the narrowest point. You may need to split the road if there's a distinct narrow section. Adding lanes=1 and passing_places=yes or passing_places=no (as appropriate) may help, together with the positions of passing places as highway=passing_place nodes.

Capturing the smoothness=* values for potholed sections may help. I find the StreetComplete app (Android only) is very good for this. You can split a road in the app if the smoothness changes significantly along the road.

If there are red triangular warning signs (and I appreciate that there may not be, due to Cornwall CC's budget constraints), you can tag both the sign and use the appropriate hazard=* tag on the affected section of road. See:
https://www.legislation.gov.uk/uksi/2016/362/schedule/2/made
https://wiki.openstreetmap.org/wiki/Key:hazard#Traffic_hazards

If the three parish councils in this area write to some of the satnav providers like TomTom, that might help.