Is it not the case that the club face actually remains square to the swing plane and just appears to be closed pre-impact, square at impact and open after impact because the swing plane is inclined from the target line?
Good question and I think that answer depends on how the club gets released. Before Tathata training I would have the club face as open as I could through transition because I was rotating my body through the swing and using forearm rotation to square up the clubface. In that kind of release having a closed clubface would result in severe hooks. However, if you look at DJ, he is very shut coming into impact, but because he tends to pull the club through impact without any forearm rotation it stays square.
You are on the right track in that from our vantage point we are looking down on the swing and thus it appears as a straight line but in reality the plane is three dimensional rotating around our dantien.