I actually was around back then and was in the stands for each of the national championship games Jimmy and Schnellenberger won..
And btw the answer is yes to both your questions. Yes Howard thought it was a career advancement. In addition to HC and GM duties he was given a small ownership stake in a new pro football team. He has said multiple times he regrets ever leaving Miami, And yes it was a no brainer for Jimmy too. He and Jones were still close friends back then and the Dallas Cowboys are the premiere franchise in pro football and maybe all of US sports along with the Yankees.
Neither was any, shape or form forced out,