It's no good, you're arguing with someone who is deploying the last-gasp* "who can say?" tactic of denial. No evidence sufficient for court? The GOP's lawyers themselves say they're not alleging fraud? Republican officials working in the system that need to be complicit in a fraud against their own party? None of that matters- all you need is to imagine a scenario that's maybe as plausible as anything evidence could show or logic support, and, voilà- "who can say?" It's irrefutable because it works against the concept of evidence or logic, even against the concept of refutation- it's unanswerable because that's its design and only purpose.
*Or, as Abe Lincoln might have said, "down to the raisins." A little girl makes herself violently ill by gorging on raisins, then piling a bunch of other junk on top. After a prolonged bout of vomiting, she said she was knew she was going to be better soon, because "I've got down to the raisins." Somehow, that little story seems very appropriate here...