I think this is an important point. Although Robson should have been the ultimate decision maker on hiring for the Football Department, the extremely close relationship between Hird & Evans should not be forgotten (they even attended a session on AOD9604 together, allegedly investigating investment opportunities), nor the fact Robson was an outsider.It sounds more likely given the facts we have, that Evans gave Hird a lot of power as coach.
It is not too hard to imagine Hird using his relationship with Evans to go over the head of Robson in gathering the people he wanted around him. Robson could hardly go to the board & tell them he thinks Hird has too much power. You are talking a 3rd generation Bomber v an outsider. We know who would win that contest.