You're mistaken about a couple of things.
1) Despite the pro-rata clause, the Big 12 has been in talks about adding UConn. So the contracts do allow it, the money just has to be negotiated and isn't guaranteed. It makes those additions more difficult but it's not true to say it's a "fact" that the Big 12 couldn't add MWC schools.
2) With regard to the PAC, if all 10 members stay put, I wouldn't expect the PAC to pursue any MWC teams besides SDSU. However, if Arizona and Colorado joined the Big 12 and the other members stayed put, they'd almost certainly look at some of the other MWC teams. Fresno State, UNLV, Colorado State, and Boise State could potentially prevent themselves from getting one of those spots and the PAC would likely look east for partners (SMU, Rice, Tulane, UTSA, etc.). These schools don't want to miss their window because they were being petty towards SDSU.
3) I don't think you understand the GOR thing. You keep mentioning the "99 years" as an extreme example but it's not just the length of the issue. The MWC already has a GOR through their next media contract. They've also outlined penalties if a school breaks that GOR. Those penalties have already been discussed in this thread. What I think you are advocating for is a more punitive exit fee than the one that was previously agreed upon. I've already explained to you why teams would be reluctant to sign this. Look at the ACC. After Maryland left, they signed a GOR with an extremely punitive exit penalty. The top half of that conference deeply regrets that decision and have teams of lawyers trying to find a way out of it.