The "USPS whistleblowers" story is probably more sophisticated than the average person would understand. There are a lot of legitimate reasons why political mail would go undelivered... and some illegitimate reasons.
POLITICAL MAIL: During the 2020 election, at my office we had to separate all the undeliverable political mail, and we wrote on the mail piece why it was undeliverable. By us doing that, if they wanted to, the Postal Inspection Service could actually go back and verify that the place the mail piece was addressed to was vacant/no mail receptacle or whatever. Whether the inspection service actually did that, I have no idea, but I don't doubt that they do it. This is not something that happens with all political mail, though.
All this shenanigans could easily be avioded, though, if these PACS and campaigns would just pay for forwarding service. They send their mailers out with the cheapest option available to them, and the USPS does all the extra handling of political mail for free. If they actually paid for forwarding service, any undeliverable mail would be forwarded, or returned to them... but I really doubt they want to go through the trouble of taking back all those undelivered flyers and verifying for themselves why it wasn't delivered... that's too much like work, and who wants to pay more and do more work?
BALLOTS: so, at every location there might be 15 people that "handle ballots at each location" but 15 people will not "handle each ballot". Obviously if you have 10 carriers, 3 clerks, 1 supervisor, and the Postmaster- that's 15 people handling ballots, but the only people that should be touching YOUR BALLOT is the carrier or clerk that took your ballot, and they will give it to the Postmaster, or whoever is collecting the ballots to turn them over. I can't speak as to what happens to the ballots after they leave the office, though.