I think this is all UO leaving no stone unturned in trying to get out of the Pac10. I think B1G is just being kind, while also knowing this destabilizes Pac10 more without it looking like they are the bad guys. Although maybe they don't care, their Alliance sure didn't slow them down going Cassius with USCLA.
First, I don't think UO goes alone in any scenario - UW or Stanford goes with.
Second, UW and Stanford are both better choices for the B1G than UO. Their advantages in TV markets and academics over UO are more valuable to B1G than UO's current athletic (football) advantage.
Third, it's more likely B1G takes FOUR (ie UW, Stanford, Cal, UO) than just UO.
Last, Colorado might even make more sense for B1G than UO. I don't have a strong opinion about that, but I do think its a non-zero chance.
As far as the Big12's priorities...
I think the best fits are the Mountain 4, period. UW and UO are always going to want to be in the B1G, and until the B1G says "no nay never" they will wait. OSU and WSU are too dilutive. Cal and Stanford likely would rather go Div3 or independent before joining B12. I'd stick with the 4 and leave room for ACC teams whenever it gets poached.
It feels to me like AZ and Colorado are going to petition the B12, which will let the B1G be the "good guys" and pick up 2-4 Pac12 schools. Then ASU and UU go Big12 too. Leaves both Big12 and B1G with 4 more spots for ACC teams in future to go to 20 and 24 respectively.
The B10 will take an even number (0,2,4), otherwise they’d have to either go to 10 or 8 conference games.
2 - WA and Stanford or Cal. Sure, OR has more on-field success, but the B10 makes money off of TV markets (unlike the B12) and Seattle's MSA population is roughly the size of Oregon. The only reason Stanford and Cal are serious possibilities is because of the Bay Area market. You don't need both to get that, it's redundant. Cal only gets in if the CA board of regents can get them to ride UCLA's coat tails, and that's a pretty big if.
4 - WA, OR, Stanford or Cal and TBD. That leaves one spot, and the B10 would probably go for more markets, meaning ASU, CO, or Utah in that order.
6 - Getting to 4 for the B10 was starting to reach, so six would be very, very surprising.
As for the Big XII, adding WA/OR if available would be a great get. Yes, they will be pissy about having to join the Big XII and yes, we did just part ways with the pissiest school out there (Texas). But realistically, they wouldn't have the power that Texas did simply because they could not join whatever conference they chose to like Texas could, otherwise they wouldn't be in the B12. Yes, they'd act like they were doing us all a huge favor and might try to throw their weight around, but at the end of the day it would be eminently clear that if they decided to leave, they'd have nowhere else to go. Everything I said about OR and WA would be exactly the same for Utah.
The B12 probably only takes more than 4 only if both OR and WA are available. That might mean one of the corner schools gets squeezed out by WA/OR if the B10 only takes 2 and it's not both of them. It would make far more sense to leave one on the table rather than take Cal, OR St, or WA St to even up the numbers.
Any of the remaining PAC schools would be a massively better choice than Memphis, SDSU or any other G5. Yes, Cal/OR St/WA St are far better than any G5 school. Hard rule: no G5 additions until after the ACC blows up. They're simply not worth it and not necessary for the Big XII. Sure, SDSU is in a big market, but that's not how the B12 makes its money. San Diego is home to 3.3m people, but 3.2m aren't aware that SDSU plays any sports. If I were a betting man, I'd say less than half the San Diego population could tell you what the B12 was, name any schools, or point out Iowa, Kansas, or Oklahoma on a map.