Good write-up. A few points:
If Boise loses to UNLV, Boise will drop like a rock and not be an at-large team. They will only be ranked around # 10 this week, as it is. Also, UNLV will get the P5 AQ invite but not jump the Big XII Champion.
However, if Clemson also wins the ACC, the Big XII Champion could move into the 3 seed spot (if UNLV wins the MWC) or the 4 seed spot (if Boise wins the MWC).
I am not concerned about at-large spots at all, as ISU will not be in cosideration if they lose the CCG, and I do not think either BYU (10-2) or Colorado (9-3) have enough juice to poach the final at-large spot from Alabama / South Carolina / Miami / SMU, etc. BYU should (win @ SMU) and Colorado could (Heisman Winner / Coach Prime, etc.), but I don't think they will jump those other teams.
If for some reason, the Big XII Champion does not get the Fiesta BYE and is the 5th AQ, I think there is a good chance that they get the #8 seed, so they can host a First Round Game. Of course, that would be just too damn logical!