I don’t know about the last question re homeless, etc.
Imagine a pile of ballots delivered to the same PO box, filled out by someone who has a copy of the original registration card, and are then submitted.
Of course this can happen with absentee ballots also.
The person with the copy of the registration card can request the absentee ballots and forge the signatures.
What happens to a ballot delivered to a vacant residence?
In person voting with a government issued photo ID, and absentee ballots with a valid reason for not being able to vote in person, are much more dependable and safe.
It's funny how India and Mexico with government issued ID, with fingerprints have their act together, but we in the US don't.
Of course as long as the Dems keep screaming voter suppression and all votes count (including illegal, deceased, and multiple voting), we will never have faith in the outcomes.