In the current system people can intimidate you as much as they like but they can't follow you into a booth. They can't inspect your ballot after you've marked it.
Only you know who you actually voted for. That's the point.
Unless, of course, you are asked to provide evidence of your vote with the camera you are likely carrying with you into the polling booth.
I'd guess mail-in ballots increase participation at the expense of some coerced ballots. Surely the net outcome is positive. On top of that, it helps to remove the chance for the use of intimidation at the polling stations. I remember that being an issue last election.
Somebody else mentioned that some areas have a system where if you show up in person, your mailed ballot is invalidated. That seems like a good safeguard.
The bottom line for me is that I think the option of mailing ballots increases participation. There may be an increased number in coerced ballots as well, but I think the net result is postitive.
Filming yourself could easily be prohibited. In France, I'm pretty sure that the voting officer wouldn't let me put my ballot if I was filming myself doing it.
Canceling mailing or e-voting ballots when voting in person can be countered by requiring that the person hand over his ID card for the vote duration.
The idea that a vote might have been bought is a dangerous one. It's dangerous even if votes buying are not proven. I'm still not convinced about e-voting for this reason. There should be absolutely no doubt on the sincerity of a vote.
Only you know who you actually voted for. That's the point.