I think that the names of the positions and the duties of each particular name are being over-emphasized. Every club in the world has essentially the same set of things they need to accomplish to be successful. Players need to be scouted, evaluated, bought, signed, and released. Players need to be taught how to play football, obviously more when they are younger. The various teams need to have a coach who works on selecting the players, motivating the players, organizing the team, in game management. These are the same for any club. As long as these things are being done well the club will be successful.
The thing is that the set of these jobs in modern football amounts to more than one, or several, people can do by themselves. There are all sorts of arrangements of responsibilities that can be made to essentially achieve the same thing. You can have a manager who has ultimate responsibility for the whole program, like Ferguson or Wenger. They will still have scouts, academy people, coaches, etc.. You can have a Director of Football who does nothing but oversee the whole program, with a series of experts in different areas, you could have a manager who oversees the first team and first team transfers but doesn't get on the pitch to train the team. You could have a first team coach who is told what the system will be, the way of playing, and someone decides which players he gets for that system.
It doesn't matter very much what the system is, as long as there is a system that is understood and agreed to by everyone. Within that system you need experts doing what they are good at, and not doing what they are bad at. There are massively successful teams where the manager is the person ultimately responsible for everything on the football side (Man Utd). There are massively successful teams with a Director of Football system (Bayern Munich.) There are massively successful teams where the system of playing, set up by someone who doesn't even work at the club anymore, is viewed as the ultimate responsibility (Barcelona).