Just as a datapoint this doesn't exist everywhere. In France for instance it's not possible to vote without physically going to a polling place unless you're living in a foreign country and only for certain elections (not possible for the presidential elections for instance). It is however possible to give a "procuration" for somebody else to vote for you but you still need to go to the police before the vote to make the procuration so it wouldn't be a very convenient way to fraud since you'd have to coerce the person into giving you their vote in front of a police officer.
I agree that if your electoral system accepts mail-in ballots then other electronic ways to vote remotely don't seem that much worse overall since they would share pretty much the same potential for abuse. I guess paper ballots would make it a bit harder to automate fraud at least.
In Australia, early voting can be done at polling places for several weeks before the election (though there's less polling places open obviously). Mail voting is also provided, but I agree with you that it really is something that has questionable merit -- when you're overseas you can vote at the Australian embassy. The only real use-case it serves is people who live in the outback or are worried about personal safety (who are "silent electors" -- where your address is not posted on the public voter roll), and I think that mail voting should only be provided to those people.
It's an interesting idea, though how would you maintain the secret ballot? If you ask them to make a "negative vote" they can suddenly vote three times without any chance of being caught.
Agree on keeping votes sealed until live polling closes. But there is no need to check every mailed-in ballot against in-person votes.
Instead, when a mailed-in ballot arrives one marks the voter rolls. When a voter checks into a polling place, the rolls are anyway consulted. At that time, if a live voter shows as having already voted: that ballot is found and destroyed, and the voter given a fresh ballot to cast.
This is also a problem with mail-in ballots. (It’s why I believe early votes should be allowed to be overwritten by in-person votes.)
The issue with smartphone voting with selfie verification is almost everyone’s phones contain selfies. (Americans overseas are also likely to have a photo ID on their camera roll.) So controlling the phone means controlling a vote. That’s a scalable risk.
Moreover, if the app is anonymising after verifying, it means the app developers have been deputised by the Board of Elections to verify voters and count their votes. (Whether you voted is public, in America. For whom you voted is not.) That’s a big deal!
The blockchain add-on is worthless if done right, dangerous or illegal if done wrong, and increases the cross-section across which the above issues get amplified.