1) He isn't managing their money, but dumping WVU to the curb would have a massive financial impact.
Fiduciary Duty (per Cornell Law) "When someone has a fiduciary duty to someone else, the person with the duty must act in a way that will benefit someone else, usually financially.
2) The Big 12 is an organization but it is not owned by any one person. It exists to serve its member teams. The B12 commissioner works for the schools in a way similar to the NFL Commissioner. In the end, if everyone has a home going forward and are satisfied with the outcome, then that is all that matters.
There is a real possibility that every team ends up elsewhere else. If Luck can help maneuver that, who is going to sue him?
Lastly, if a couple more teams leave, then the B12 no longer officially exists. Kinda limits one's legal options when there is no longer a conference.