No signatures where I live, what's next analysis of hand writing? You only get one mail in ballot, which is returned absolutely anonymous. Once you order one, you are struck from the on-site ballot list. You can exchange your mail-in ballot, I think, for a normal paper ballot. if you return the mail-in one. So your solution would mean manually following up every single mail-in ballot and steal it. Assuming you find out who ordered one.

using the "left-over" ballots of people not voting, sure, all you have o do is to convince the other 3 to 4 people present at the polling station to go along. and since we have literally thousands of those stations you have to repeat that a lot. And as soon as the participation exceeds the other places, people will investigate. The provisional count done on-site is redone before it is official, so again deviations will be found. And if they are not, congrats, you managed to stuff maybe a dozen ballots, if you are lucky.