No coach would ask his CB pair to split in any circumstances when their team doesn't have the ball.I noticed that too but I'm pretty sure that was the way they were asked to play, covering for the full backs who were bombing forward. Whenever it happened Romeu dropped back to fill the gap
CDM can drop in the hole when you have possession but without it that is suicide. I think it was just lack of an organisational presence. A player like Virgil is not only good because of his ability to defend but also because he improves other players. Neither Maya or Jack are the type to take control and make the defence more compact etc.