If there are signs indicating pedal cycles on the south side and pedestrians on the north side being permissible, they are illegal due to the legislation (and they would be far from the only invalid signs on UK roads and paths).

It's part of the NCN because National Highways allowed Sustrans to put their signs along it. NH stopped enforcing the 1996 legislation in full, but that doesn't mean it doesn't apply.

Official truth can be prioritised if there is community consensus, as with the A8(M) near Glasgow.

I believe the current signs are incorrect.

"Changed motorway-committed roads to highway=motorway as is standard" - do you have anything to back this up? Sections like A14 Catthorpe - J1 would be labelled motorway if this were the case

https://webmaps.hertfordshire.gov.uk/highwaystraffic/index.htm

Check 'Primary Route Network' in layers

The M25 should be tagged access=no as motorways are special roads where classes of traffic are 'whitelisted' for rights of way on them. the A282 shouldn't be though, I'm not sure why it was in the first place.

Incidentally it didn't work the way I hoped so I wouldn't bother trying again anyway.