I think people use middle and edge as a defined role.
These days they are more often than not intertwined. Players tend to play where they are best served.
I read an article several years ago on another forum that explained it very well and it was how I was coached.
He used the terms corridors and attacking shapes. As far as shapes go, I know most people are aware of the linking role locks play, no better performed by Yeo.
However corridors are like drawing lines from both posts, end to end. That traditionally describes the middle However the goal posts change as they say. So to add to that, middles are not so much restricted to centre field. Haas has often ran at an edge (the corridor either side of the centre one) trying to tie up the cover.
Fafita is listed as an second row/edge but spends most of his attacking out in the centres then moves into more of the centre corridor when defending. He's big and fast enough to play in every position

So ref to your good post, the only thing I'll add is your reference to his D.
I think he's come on leaps and bounds defensively. If our season was over, I'd be playing him there to learn the trade.
I think he's a natural ball playing lock like Yeo but has the advantage of being quicker and has an awkward running style which was beautifully on display on Sunday. He sort of gives us three ball-players.
On a dry track in the past, AD has been known to ball hog and burnt on the outside. This is weird considering that once he gets going, he's one of the quickest players in the league. Closer to the middle/edge, this is not an issue, we need that run first mentality and straighter running.