The SEC meanwhile plays 9 conference games + one Big 12 game + 2 buy games , since the B1G/Pac/ACC aren't playing them anymore. ESPN and the leagues decide on the SEC/Big 12 opponents less than a year in advance to maximize the quality games (but Bedlam being one that always happens; Mizzou plays KU, KSU, ISU a lot; Texas and A&M play Tech/Baylor/TCU often).
One, I don't think they're not going to play the SEC any more. They're just going to agree on the rules they want. If the SEC doesn't like them they can negotiate or go it alone. I don't think anyone here is trying to distance themselves from the SEC, just ensure the playing field stays relatively even.
Two, the way things are going I'd imagine any scheduling agreement would be 2 games, whether its the SEC/Big12 or the "Alliance". If the concept is raising media revenues, you need home games for that. 1 game only gives you half the games. With two games the Big 12 could sell 8 SEC games a year over 4 and increase ticket sales in the process.