The reason it's a silly question to ask has nothing to do with whether the Big 12 should / would welcome Nebraska if they desired to return. The Big 12 would bend over backwards to make that happen if it was a realistic possibility.
The issue is that Nebraska has next to no reason to do it, as they are significantly better off in just about every way in the Big 10.
You mentioned it earlier, but I think the academic reputation of the Big 10 is a huge factor. Every one of those Big 10 schools is pretty elite in that regard, pretty good company that UN probably wants to keep. No matter how bad some of the teams might suck at football, there are no Texas Techs, K-States and WVUs in that conference.