In Australia, we do a similar check, but using paper rolls and cross-check for duplicates afterwards. No computer to go down means no delays. If they find a duplicate they investigate, and if there's ever enough to make a difference, they would re-run the election in that area. Once we lost a box of votes and they had to re-run the election across a whole state to make sure.
2. See #1.
3. When people vote by mail, they get ticked off in #1. This doesn't happen immediately, because mail-in ballots can be claimed lost or found to be spoiled (with opportunity for correction), and because they have to be cross-referenced against in-person votes - which is why it takes days or weeks for all ballots to be tallied.
People running elections aren't stupid. They've already thought of all this.
Unfortunately, the barrier to making false or completely unsubstantiated allegations about wide-spread voter fraud is zero, so every Tom, Dick, and Harry is currently throwing in their two cents on the matter. (Bonus points if they don't actually understand, or actively pretend to mis-understand how the process currently works.)
I'd give those claims the time of day when their proponents are ready to put some skin in the game, and swear on them under penalty of perjury.
That's like saying a shotgun can cure cancer. The tradeoffs required are so laughably bad your solution is not worth considering.
1. Central database of all citizens. (Can be state-level if you want to do that). 2. Require voter-ID to link against 1 above. 3. Tick off people on the database as they've voted so they can't vote twice.
And just like that, most (but not all) of the "mail in ballot" concerns are rendered mute and we can all continue with trusting the election results.