My guess is that the old SWC school don't really want another Texas school in the mix. My bet is that they'll go for the likes of UC and Memphis. I also understand that at the time WVU joined that they stated their desire to have a few schools between them and the other B12 also join. Right now they are kind of isolated with no natural rivalries.
There are only four schools from the old SWC currently in the Big 12: TCU, Texas Tech, Texas and Baylor. I don't think they give a flip about UC or Memphis. I bet they would love to see Houston so their Texas athletes and money stay in state rather than travel to Ohio or Tennessee.
The only logical reason for adding a Memphis and UC, as you've stated, is for the benefit of WVU. It just so happens that those two programs have been good lately and have made moves to get to that level. The better fit though is still Houston.
Someone else said they would love to see them go to 14 teams. If that were the case, I guess you throw UCF, BYU, Boise State and USF in the mix too.