States Legislatures are given full authority to determine how their electors are chosen, although the date for selection is set by Congress. Clearly Atlanta and Philadelphia invited more people to vote than were legally allowed to vote. That is agreed to by all sides – rules were changed without changing actual laws.
Georgia, Pennsylvania, Nevada, Texas, and so on need to update their manner of choosing electors. I don’t think the amount of time between the popular vote and the date electors are certified is anywhere near enough time to even begin to investigate fraud. Yet states cannot change the time between these two events. Or can they?
Let’s imagine that Arizona moves the selection of electors to the Legislature. That is, on the first Tuesday after the first Monday in November the Arizona legislature will meet and vote on a slate of electors. I suspect the people of Arizona might be royally po’ed about losing their say in the vote. But what if back in August there had been a popular vote of who the citizens would like to see elected? That is, the August vote informs the legislature without being legally binding. This process would be allowable under the Constitution since the selection is still nationally uniform. Also, if there is the appearance of widespread fraud (looking at you Atlanta) the state legislature would likely vote in a partisan manner rather than a unified manner. Critically the executive branch gets no say whatsoever in this process. There is actually no ability for fraud to overturn the election, since the actual election is by the Legislature.
But what about the cost of running two separate elections (since you can’t shift the date the Senators or Representatives are elected)? Don’t care. Election integrity is worth the cost. Also, the cost is probably small compared to much of what the state government does.