Kelly was hired 11/29. The Joseph move wasn't announced until 12/3. If he wanted to stay at LSU, he had plenty of time to make that happen. As you said, I can't imagine that Kelly wouldn't have retained him if he'd wanted to stay. Looks like Joseph decided he'd rather jump on a sinking ship at Nebraska than coach under Kelly.
As you said, it seems stupid to go into a one-year situation just because it's your alma. Then again, I thought Frost was stupid to take that job, which he only did because it's his alma. His is a warm weather system that requires the type of athletes you get in the South and on the West Coast. He'd scare me at a place like Florida. Nebraska needs to get back to brute strength option-oriented steroid football.