I think he should open up his S9 and re-arrange the boards. If the new arrangement results in chain #4 again running hotter, that should definitely prove the defective BMT engineering.
The hotter board caused by the lower airflow there is not 'defective engineering'. It was an accepted
design choice. Since the boards in that location do work acceptably well then form-factor overrides perfection.
Perfection is not only unneeded for most things, it is also usually very expensive to build. Part of Engineering is learning the very gray area between, "Well, it
works" and "it works perfectly in all ways". Most often the final product will fall somewhere between those two points. For consumer goods the roadblock on the path to perfection is usually Marketing ideas of what that (perfection) is and cost.
Now if we start seeing failures of that board more often than the others, then you have a case.