I believe there may be points that are overlooked a bit, and I believe some of your lab pictures support.
The various scrap materials that form the original bars or 'alternees' are forge drawn to some length. Point being that any inclusion or flaws are reduced in cross section and aligned lengthwise.
The process of layering and twisting dilutes and prevents any one flaw from influencing integrity of a final barrel. Forge welds are very different than today's stick or wire welds, and one would think the final possible weak point, or welds between the ribbands, would be well tested during proof.
Thanks for taking the time Doc Drew. Merry Christmas.
