"<i>What if you buy the early bird check-in the night before your morning flight?</i>"

It doesn't work that way. There is an earlier cut-off (I don't remember exactly how far ahead but it will be on the Southwest website)

Plus, at exactly 24 hours pre-flight, all the non-business and non-early bird passengers will start checking in. So all the A and most of the B group would already be filled - so that wouldn't work even if there was no cut-off.