Consider the following British-English sentence:
It is difficult for the decision-makers to believe, let alone accept(,) that the increased hardware-security would outweigh the production loss caused by security-enhancing measures.
Is the second comma mandatory, forbidden, or optional? In the optional case: what shift in the meaning does the introduction of the comma (versus its absense) cause? Justification supported by references would be welcome.
(If it has any relation: we use the serial [Oxford] comma throughout the rest of the text.)
OOD provides a single example without a comma:
Who on earth would be prepared, let alone equipped to take on such a challenge?
But, there is no explanation (and it might even be a typo), so, all bets are off.