As KY Jon says above, wall thickness has no bearing on proof other than if it is thin enough the wall could fail, rivel or bulge which would of course mean it fails the reproof test.
No proof house has ever measured wall thickness as part of the proof test. They have a recommended minimum but that is all it is: recommended. They do not make any comment about the wall thicknesses and no measurement takes place BEFORE the proof test. The tube is 'viewed' for imperfections, pitting, bulges, dent etc and the chamber is checked to ensure that it meets the very exacting dimensions for its nominal chamber length and rim dimensions.
Bore diameter is measured after the successful proof test to establish what bore size is to marked on the flats.
Striking off the outside of the barrel wall does not effect the gun's state of proof.
The auction houses often undertake small amounts of work on guns submitted for sale, including arranging reproof, but at the clients cost and risk.