(Still wondering about that last thing if anyone has any insight!) (But also -- )
Looks like we won't be using Direct Add after all, they want to approve subscription requests. I mean, the eeeeeasiest thing to do is use a webform on our website (it's Drupal so there's a webform module), send the webform submission to *both* the "+owner" and the "+subscribe" email addresses, ...and that's seriously it, we can collect whatever info we want in the webform (which they only want to have to check against their own records of who's a member of the community IRL), and the user will have a membership request pending approval.
The automatic email from the webform submission is like, sort of from the submitting user's email address, and in my testing, it seems to work great -- but the Display Name isn't getting populated, and I can't figure out why. But that's probably out of scope for this thread -- I know I started the thread, but still :) I'll copy into a different thread. (But/And still would love advice on the username/password question I posed in my previous message! :)