You make excellent points.
I think if he had reverted to type ala Ole or Mourinho after injuries to key players in the build up phase, and gone back to the double pivot in a more conservative play style, it would hide the deficiencies of some of the players who naturally struggle to defend higher up the pitch in 1v1s. And I don't think we would learn anything from that as far as transitioning into a more dominant team, but it would (imo) be counter productive in the mid to longerm, because it would create a false sense of security with a belief that some of the players in the backline who very obviously struggle to play higher up the pitch when the opponent's close off the space quickly via the high press, aren't the problem. And it was very clear to see that the backline wasn't closing off the space which is the absolute basics in this type of setup.
I don't believe we've come close to the football ten Hag wants to implement. I think strides have been made to change things by removing certain players with a view to playing a more proactive game, and also the introduction of the young kids into the team like Mainoo and Garnacho. But as far as how close he is to getting the first 11 to play a more positional game in possession and a higher intensity without the ball, I agree 4 players with the requisite attributes will change the dynamics of the first 11. I think just two players will have a huge impact if those players are a RCB and holding midfielder with the requisite qualities. When you improve your first 11, the squad depth will automatically improve.
And like you say the statistics will paint a very different picture with a backline stepping into areas vacated by the midfield when applying the press from the front. In that situation the space will be between the GK and the two CBs.