>For example in '2000 Mules' they propose that democratically aligned charities like homeless shelters and elder care homes would gather ballots from their vulnerable populations, fill them out for their preferred candidates, and then have mules distribute those ballots to publicly accessible ballot boxes 3 to 5 at a time.
In order for this to actually work, the nefarious operatives would need to forge people's signatures on those mail ballots. Otherwise, they'll fail the signature match at the elections returns center and the ballot will get tossed. Here's a good rundown of how that works, at least in California [1]. Do you have reason to believe that the signature check doesn't work?
Stepping back, rigging elections by stuffing mail ballots makes zero sense from a cost/benefit perspective. It requires massive amounts of effort to coordinate all those people so the plan proceeds undetected. A rational attacker would be better off taking an opposite approach: throwing away opponents' mail ballots. That requires far less effort. I'm skeptical of these mail ballot stuffing claims because it's an overly complex Rube-Goldberg-machine of a plot.
Other than throwing away opponents' mail ballots, is there a viable attack that could actually work?
[1] https://www.youtube.com/watch?v=-YJyQbckMDw