Re the Coleham Head "tunnel" -- I wasn't aware that I had! Whether it was previously wrong or an accidental change by me I don't know, but it's a bridge again now. Will answer your other points later (hopefully, this evening).

But OSM is surely far more than just some kind of navigation app -- and in any case no-one is "directing" map users to take (this part of) the B4385.
That this section of road is a Class II highway and that it is a part of the B4385 are facts verifiable from public (non-cartographical) sources.
Are we saying that these are truths we must conceal because Powys CC has erected no road signs bearing the information in question?
If so, then we must surely do the same for the many hundreds of kilometres of roads in London (let alone the rest of the UK), the classification and numbering of which are not indicated "on the ground".

>> Llanfair Caereinion High Street as being part of the B4358 <<
Oops,my butter fingers. Should read "... part of the B4385"

Hi, Phil.
Whilst it is true that this stretch is not signed with the B4385 number, I strongly suspect that this is one of those cases where the HA has deliberately provided minimal directional signage in order to discourage through traffic.
OS mapping has shown the section as having formed a part of the B4385 since at least 1953 (and as a Class II road since 1946),and continues to do so. Bing Maps, Defra's MAGIC maps, GoogleMaps, Street Map, and Via Michelin all show the number too (yes, I know that some of these sources do feed off each other!). More convincingly, documentation on planning and conservation matters published by Powys County Council (the HA) on dates up to and including 2018 (currently the latest I have) refer to Llanfair Caereinion High Street as being part of the B4358, and similarly with points further west -- e.g. the "junction of the C2130 and B4358" at SJ096065.

My oversight! Thanks, Mike. Now corrected.