Granted that neither of these scenarios is the best case or even close to it
Scenario 1: BYU makes the playoff... but other than that it's about as ugly as can be. Something like losing to ASU, things falling right to make and win the CCG, and then getting obliterated in a road game at Ohio St or Oregon.
Scenario 2: BYU misses the playoff, but the Big 12 rep does well. Say it's Colorado or Arizona St. They get the #4 and a first round bye, beat Ohio St/Oregon in round 2, then the other one of them in round 3 to make the final where they end up losing a close game to Texas.
Is there any point where Big 12 (non-BYU) success is worth a little less BYU success?
My gut feeling is that's not the case. BYU to win the conference and make the playoff is more important than any level of conference success