For close races, the parties get those lists of whose ballots were rejected, check against their list of known supporters of the candidate, and will “chase” those signatures (contact the person and explain to them how to fix the problem). They’ll door knock with the correct form for the person to fill out if necessary. I’ve done it,
In my California county, San Mateo County, the last election was an all mail-in ballot election. It saves the county a ton of money. You could go to a “Voting Center” to vote on a machine, but there were only a few Voting Centers scattered around the county. I think they purposely made it a pain to vote that way to encourage mailing in your ballot.
I’ve been voting by mail for about 10 years and always check the county election website a week after mailing it in to make sure it was accepted. There’s never been a problem.
In the very close California 21 election, friends of mine were “chasing” rejected ballots. I think it involves calling the voter and knocking on their door to persuade them to go to the county to verify their identity. Seems like a reasonable thing for candidates’ supporters to do in a close election, and nothing like the alleged crimes in North Carolina. People’s votes should count if they cast them legally.