OP edited their post, but they were talking about methods to change maybe one or two dozen votes at a time. The minimum number of votes that you would need to reverse the 2016 presidential election was 107k. That was if you had perfect polling going in and knew exactly what votes to change. In practice you would likely need to change several hundred thousand votes if not over a million. This process would need to be done in multiple states all over the country. You would need a large distributed team of people all committed to defrauding an election. That isn't a smart plan for a conspiracy.

No one here is even considering a nation state working to change the outcome of an election. A spy network might get away with it.

Online voting could be vulnerable to such an attack but paper-based voting such as is used by mail in voting is very low tech and distributed. hacking and fraud are hard to scale in that case.

So, every mailbox in a post office gets one or two voter applications or ballots. A bunch of them get sorted out and thrown in the lobby trash can as people pick up their mail.

Some unscrupulous person grabs a bunch, forges signatures on them and mails them back in.