The addition of CSU to the Big 12 would allow the Big 12 to keep a portion of the Denver TV market. Regardless of their fanbase, facilities, admin support, alumni support, on field success or any other measurable benchmark, the fact that CSU can probably deliver a piece of the Denver TV market makes them #1 on the Big 12's expansion list, IMO, if CU does leave.
No chance CSU is admitted to the Big 12. Conference members have to maintain certain minimum requirements, such as football attendance. I'm not sure what the minimum is but I bet CSU isn't even close. Even Baylor has an average of almost 15,000 more people attending each football game. They just don't have the resources to be a viable member.