Hinckley Yacht Services sure sounds like a professional yard. Funny that they did not have the correct size deep socket needed to "check" the torque on the nuts. How does a full service yard fail to have a common tool? They did have a torque wrench, but nobody seems to know what the correct torque specification is. I question the need for a torque wrench when the specification is given as "about".
It is also my belief that it would be impossible to check the torque on nuts that have been tightened and subjected to bilge conditions for almost 30 years, without first removing the nuts, then cleaning and oiling the threads. Aside from the normal corrosion you would expect, there might also be some galling of the stainless steel. Any of these conditions would render a torque check useless, and I believe it is responsible for the snapping of the stainless threaded rod. Looking at the pictures of the "snapped" threaded rod, it is obvious that the rod failed exactly at the point of the contact with the nuts where the maximum force was applied.
I am neither an engineer or a metallurgist. I welcome any comments as to my assertions. It is also my belief that Hinckley is negligent and caused the owner a $7000.00 repair. I find it funny that there are no pictures provided of the original threaded rod, but there are pictures of the snapped threaded rod with nuts attached that conveniently do not show the condition of the threaded rod. I think it would be helpful to see what the actual condition of the original threaded rod was, and it would even be more useful to have a sample of the rod tested for strength.