Envoy for the regions and nations is obviously a demotion, but how much of a demotion is it?

I think that depends how serious the government is about devolution.

Yeah, is it a genuine concession to fleet street or just a feeble feint? Can't see the long term benefit either way.

Chief of staff and envoy for the regions and nations aren't well-defined roles (especially the latter) they're very much as powerful (or not) as the person doing them makes them