It seems to me that the only thing your experiment did was to prove that a fouled barrel when enclosed in a sealed container with H2O2 covering part of it will rust above the fluid and below it.
As it was in a sealed container, some of the H2O part would have evaporated making the humidity in the air space 100 %. Any of the extra oxygen that escaped from the H2O2 solution into that 100 percent humidity would have added oxygen to the air space.
If the test proved anything it seems to be, 100 percent humidity plus some extra oxygen rusts steel coated with black powder fouling just about the same as the area of the barrel that was below the fluid level.
Perhaps a second test could be made that duplicates this test but, instead of using hydrogen peroxide, substitute distilled water.
It would be rather important to make sure the distilled water was
not aerated by shaking it up because that would introduce air (read oxygen) into the solution.
Of course the trapped airspace above the water will become a 100 percent humidity zone and it will very likely cause the same sort of rust your experiment shows.
What will be interesting to me is, will the non-aerated distilled water cause rusting below the fluid level?
If it does not cause rusting, I think it would go a long way towards proving or disproving that oxygen, either in the air above the fluid or the fluid is the culprit.
Putting it another way, if hydrogen peroxide causes rusting below the fluid surface and non-aerated distilled water does not the presence of oxygen in hydrogen peroxide could be harmful to a steel barrel.
Just a thought.