I looked online for a mailing date for paper forms, and couldn't find a firm date. I did find that coronavirus concerns have pushed most dates out a few extra weeks, so the new deadline for responding is August 14.
My first letter with our ID for responding said "if you don't respond online, we will mail a paper form in a few weeks". The mass mailings are on a rolling basis, not all on X date. We received a second letter for a former rental apartment in our house, no longer rented, a week or two later. Then received a postcard reminder for the first letter (though I'd already responded online) - IIRC, that's expected, the first follow-up is blanket just like the first letters. But later follow-up should be targeted to nonresponders only.
I responded for the unoccupied address, too. There's an option to report no one lives there now. I wanted to make sure no enumerators had to visit us to straighten it out. Very easy.