Every ballot should have a unique serial number and QR Code. Then put the scans of ever ballot online so voters can check that their ballot was counted correctly. And allow for bulk downloads so the public can count them themselves.
>election is illegitimate due to widespread voter fraud, how does that impact that downballot races?
Representatives are defines by the Constitution as being elected by the people. Senators and the President are technically selected by the State legislatures. It's just de-facto standard practice to put the question to plebiscite. They can fallback to simply decreeing the winners. So only the down-ballot races for Representatives will be problematic.
> if you know your serial number or QR code which most people won't
Take a picture of it with your phone. Or have a tear-away coupon with the number.
>Ballot images online won't work. Think spoofing or two sets of books.
It better than anything else. Enough random people can spot-check their own votes via serial number. Then combine that with bulk downloads and a public record of who voted (no record would be kept matching voter to ballot) and you have the best system possible, even if imperfect.
Of course it can be manipulated. But it will be much harder to do (more way to get caught) than our current system of election officials stating a number and expecting everyone to accept it without question.