Johnson was out recruiting the week after the season ended and was with Applewhite. If CMA wanted to force him out, I doubt he would have brought Johnson along.
My guess is that Mullen begged Johnson to come with him to Florida to get his tenure there off on the right foot and Johnson wasn’t happy enough at Houston to stay. Maybe Mullen promised him some increased role than he had at Mississippi State.
Once the championship game is played, the NCAA rule allowing for a 10th on-field coach will go into effect and a new coach will be hired. I believe Casey is the main Special Teams coach right now, along with tight ends, but it’s possible the 10th coach will focus on special teams.