The way to cheat is to make extra ballots in advance and then find nefarious ways to feed them into the machine
Wouldn't happen. Presumably the ballots are time-stamped when they go though the scanning machine, so they have to be in sequence, not too closely spaced in time, and processed only while the polls are open. Where I voted, even though I didn't have to wait, it seemed to me that an opportunity to feed in extra ballots didn't exist.
ML/NJ