So I had left this for the time being as it was not urgent, then decided to go back to it and try one more round of basic troubleshoot (as the prior "fix" seemed to have changed things) before wiping the computer; lo and behold, disabling the antivirus seems to have done the trick.

I can't say that was the ONLY issue with utmost certainty as it has been in our customer's hands for some time (so maybe a Windows update resolved the issue coincidentally), but disabling the ESET NOD32 Antivirus while running the installer as admin seemed to do the trick. I will mark the prior solution provided as the solution since that will be more helpful to anyone else running across this problem, and likely helped in our case.

Appreciate all who posted their suggestions here.