Miller,
Im not sure how we can measure the effect of the ejecta on the frame of the gun when it is hydraulically dampened by the human firing the gun. The heavier the load, the harder the ejecta will recoil the hydraulic dampener that is the human.

The force of the measured pressure within the frame and barrel assembly can accurately be measured, and will be a calculated mathematical formula, based on the propellant and the weight of the charge. This mathematical formula will not change regardless of whether the gun is held in a rest, or fired by a skinny woman, or a really fat guy.

In other words, the pressure is the only known we have to work with. Less is better, long term, when talking 100 year old guns. Perhaps the frame would do better with a load that is, all other things being equal,1/8 oz lighter, but, that may not be a given unless the gun is fired from a permanent rest. That bruise at the end of a long day of shooting is punishment the gun didnt absorb.

Another horsefly in the Tobin yogurt is the fact that the pictured gun, with the lever far to the left, and the barrels misaligned to the frame, might just keep right on keeping on. Ive never seen or heard of a Tobin opening up at the shot(LC Smith guys, Im looking at you) and while the gun might not look pretty doing it, we are not in a position from simply looking at an internet photo to conclude that it doesnt still work. And may keep right on working, for years. I wont be firing it, and, honestly, neither will you, but, that is beside the point. The barrels are always heavy in a Tobin breech, you already noted the closure bolt is heavy, and it has a substantial spring, with significant mechanical advantage, holding the gun closed.

You might be unfair in your assessment of the Tobin, with regards to it not being up to a diet of even black powder loads, but, Im guessing we really cant know that for sure from where we sit today. I have seen, and owned a few, and can only report that the 16 Tournament grade gun was in good shape when it arrived, received superb maintenance, and careful use with ammunition that was low pressure by SAAMI specs, and left in great shape to serve a new owner who clearly understood what he was getting into. Id be willing to bet the maker never contemplated the gun would be in use 110 years later.
As Dave has reported, he has never seen a lever left of center Tobin. But, he moves in higher circles than I.

Best,
Ted