As stupid as they are I don't see them committing the long-term suicide of adding four teams - fighting the pro-rata in the courts and hoping new bidders arrive by the time their deal ends.
The likelihood of any kind of promise or expected behavior (non-contractual) being honored 9-10 years in the future is slim-to-none. The networks are businesses, they will do what is best for the business (in their judgment, such as it is) at that point in time. It's entirely possible the entire landscape changes by then.
The Big XII needs to do what it feels is in the conferences best interest, but expecting some back room agreement with an executive at a network who may or may not be in the same position and with players that may or may not represent the best future partners in a changing landscape; would be foolish at best. It doesn't mean it's impossible that it happens, but thinking a hand-shake will carry the day a decade down the road is (IMO) a poor bet.