Multiple studies have shown that mail-in ballots have a very very low incidence of fraud and does not favor any particular party. That said, I'm going to put on my black hat for a moment.
Voter registration doesn't always involve a signature. It can be done online in many jurisdictions. So point 3 doesn't always work.
Voter registration doesn't always confirm your address, because homeless people are permitted to vote.
Mail Forwarding can be set up by a third party, without your consent. Ballots can be then be forwarded to someone wanting to vote on your behalf.
When setting up absentee voting, you can specify an address unaffiliated with your actual address.
Mail-in ballots can be intercepted before voting takes places or before they are picked up by a postal worker. Ballots can be intercepted at the post office prior to final delivery.
Coerced voting can occur if someone is able to influence someone else's vote.
With the exception of point 5, at the end of the day, you're changing one vote with the penalty being a felony and prison time. And it's absolutely a lot of work. So if I were trying to change the results of the election, the best place to do it would be as a postal employee that delivers to the registrar.
Again, the rate in which voter fraud happens is so low, this should be a non-issue. Does it happen? Yes. And the last few election cycles, it's been performed by people in the party screaming voter fraud, who have been caught.