Kevin, Wow. What an ordeal. It’s excellent how you have documented this, and while this is just one of at least three sides of the story, it’s pretty clear that any Googler reading this story with ambitions of removing someone from the company simply has to fabricate to HR a story about that person, invoke a 3rd party source which may or may not be identified, and then do nothing. Sadly, once on the defensive, it would appear that the accused has few options short of a libel/defamation of character lawsuit, which is of course the nuclear option.
That said, the difficulties of running an HR organization in a company that size are unimaginable. So many people, so many conflicts, so few options to resolve them amicably. All it takes is a comment, a look, or even hearsay to enflame people today, and most don’t have the spine or character to walk their accusations back to HR, as you’re well aware.
I wish you well.