Innocent until proven guilty is a standard that’s only used in criminal court. In civil court, Cain would be the plaintiff, and he’d have to prove that the defendant (pick one) was guilty of fabricating the allegation against him, which not-so coincidentally would also affirm his innocence.
It is not just in criminal court, it’s a timeless principle of our civilization. If you don’t see that, if you think Cain can somehow prove a negative, then, as you wrote so powerfully elsewhere, “We’re so far apart in our thinking here that it’s not even worth discussing.”