Funny, all the things you listed are a coach's responsibility. It is the coach's job to make sure players know their assignments, know the fundamentals of tackling, and know that they have to run out after the marker. A coach is supposed to keep their players disciplined enough not to draw the stupid personal foul.
And you bring up a good point about accountability. Is Rex holding anyone accountable for these flaws? We know he held Roman accountable, but what about on the defensive side? Has he bench a player for poor play? We all know he didn't with Mario.