Generally all mail in voting systems let you override your vote. If someone coerces you once and you send in another ballot postmarked after the first or go to your polling location in person on election day you can override that vote.

You basically need to hold someone hostage or under total surveillance from when the ballot is mailed to when the polls are closed to avoid them just sending in their actual ballot afterwards.

With an electronic voting system the window of time you have to hold someone hostage is much shorter - simply force their vote an hour before the polls close and then hold them prisoner for the hour.

This is an interesting answer and makes more sense - still possible to do, but harder.

At least here in Denmark mail voting does not just mean that you send your vote by mail.

It means that you before the voting date go to a public office, a consulate in a foreign country, etc., show you ID, go into a voting booth and votes, and they put you vote into an envelope, that is sealed and mailed to voting place.

Or that two appointed volunteers goes to e.g., a assisted living facility and witness residents voting and placing their vote in a envelope that is then sealed and mailed to a voting place.

Voting by mail is indeed vulnerable to fraud, as we saw in North Carolina last election.

When thinking about voting by mail I wouldn't consider North Carolina's system as exemplar, since it is still primarily a polling-place election. Instead, look towards Washington, Oregon, and Colorado which are states where elections are entirely by mail. Evidence of coercion or fraud is low, and engagement is high.

How do we know how many spouses are (or simply feel) compelled to vote a certain way?

I’m not saying fraud is rampant, but there’s no denying the fact that on the individual level fraud and coercion are much easier to achieve when voting is done outside the polling booth.

In many states it is possible to get an absentee ballot for any reason, which allows the same coersion potential but without the convenience of full vote by mail. Forcing voting in person also has a large effect on who actually votes (due to work or other issues getting in the way, which could also include coersion from a spouse who thought you might vote the wrong way or doesn't think you should be able to vote). Presumably people are checking if there are large swings in voting patterns when vote by mail is started that would indicate widespread coersion. Coersion can also be effective in practice even when the actual voting is done in secret.

I wouldn't recommend vote by mail for Mexico, where there is widespread vote buying even with secret ballots, but for the US it seems to me that vote by mail is likely to be more representative and increase the chance that voters will research the candidates as they are voting. No system is perfect so it is a question of what tradeoffs seem to make the most sense for particular situations.