Another poll worker here, confirming what others have said. There is a Judge of Elections at every polling place; they have to be there all day (as do several others), food gets brought in, the Judge is the one who closes down the machines at the end of the day, and announces the count while others record the information.

When you turn in your mail-in ballot, bring everything, including the envelope it came in. Like someone else mentioned, if something does go wrong, you can still cast a provisional ballot. Later, after they determine that you did not return the mail-in ballot they sent, they will count your provisional ballot. They are very serious about making sure that everyone who is allowed to vote gets to vote.

ETA: good on you for taking the time to find out for sure- this is what they mean when they say ‘make a plan to vote.’ For anyone else reading, if anything should ever go wrong on Election Day- like they mailed you a ballot but you never got it- go to your polling place and ask to speak to the Judge of Elections. They are trained to handle any last-minute questions or confusion, and if they can’t resolve it on the spot, they will almost always let you cast a provisional ballot. Again, things will get sorted out in the next couple of days, and if they find that you were eligible to vote and didn’t mail a ballot, they will count that provisional one.