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

You don’t know your history. At the time, Auburn had played UF more than any SEC team other than UGA (maybe MSU). Auburn’s main recruiting areas have always been the states of Alabama, Georgia and Florida. Auburn fought to have UF as an annual rival (and would never have fought to avoid having that game annually).

Of course the DSOR speaks for itself.