You're right, there is no way for ISU to tie Mizzou or Nebby without beating both of them which would be the first tie breaker.
Divisional Tiebreakers: The following procedure will determine the representative from each division in the event of a tie:
1. If two teams are tied, the winner of the game between the two tied teams shall be the representative
2. If three or more teams are tied, steps 1 through 7 will be followed until a determination is made. If only two teams remain tied after any step, the winner of the game between the two tied teams shall be the representative.
1. The records of the three teams will be compared against each other
2. The records of the three teams will be compared within their division
3. The records of the three teams will be compared against the next highest placed teams in their division in order of finish (4, 5 and 6)
4. The records of the three teams will be compared against all common conference opponents.
5. The highest ranked team in the first Bowl Championship Series poll following the completion of Big 12 regular season conference play shall be the representative in the Big 12 Championship Game, unless two of the tied teams are ranked within one spot of the other in the BCS poll. In this case, the head-to-head results of the top two ranked tied teams shall determine the representative in the Big 12 Championship Game.
6. The team with the best overall winning percentage (excluding exempted games) shall be the representative.
7. The representative will be chosen by draw.