re: How did the 12 SEC teams agree on these East & West permanent rivals for over a decade?

Looks like GT played at Auburn in 1904 (before SEC was formed) but didn't go back to the Plains until 1970. They played a few games in Birmingham before that (starting in 1960), but for decades all games were played in Atlanta, for the money (teams used to split the gate evenly, so Auburn made more money playing GT in Atlanta than in Auburn).

Florida played at Auburn in 1939. Played a few Auburn home games in Montgomery before that.