Right, there would only be three days a year, maximum, when hordes of Guests would likely be checking out-and-in first thing in the morning, aka 7 AM - and how badly this affected the check-in line would depend in large part on the resort size and style. POR would be more affected than POFQ; Pop would be more affected than any single All Star; ANY resort would be more affected than CSR if, as this year, that entire resort was reserved by a corporation for a conference
You can't magically create staff - if you have four people assigned to a given shift, and one of them doesn't work Sunday, you can't snap your fingers and make more appear. Even if you see the line has fifty people in it at 6:58, and you start calling your day staff to come in early, they're NOT right around the corner and they're likely not showered, dressed, fed, and ready to run out the door at that instant.