Even though everything you have done seems to have confirmed (seemingly 'beyond doubt') that the meter is telling the truth (even with new batteries), if it were me, before I started even thinking about the nightmare situation that you could be facing, I think I would probably want to see that a different meter gave the same results.
He has confirmed that. He also has one circuit (wired in different cable) which IRs as expected (>999 MΩ) with his meter.
Quite apart from the fact that, as you say, these are not the results one would expect, nails and screws through several different cable runs, with the same effect in all cases, would surely be totally beyond belief, wouldn't it?
If that's the case, and if it were a 'batch problem' (no matter how rare that might be), they might already be aware of the problem.
I think you should.
As I've said, 'unnecessary' though it appears to be, I would personally first check with another meter before I even contemplated 'doing' anything.
That might depend on what (if anything) you learn from TLC. If not only the existence of the problem but also the nature of/reason for the problem is also known, then that may give you some insight into whether the problem is likely to get worse over time - which, in turn, might influence your decisions. I suppose that, assuming the situation persists, and regardless of what you feel 'should' be done, you probably really need to discuss the situation with the customer.
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