Got around to this, at last.
Have solved all the clues,and have the "relevant question" but am still none the wiser as to 40ac or the alternative titles! Without giving the game away, is there still much to do; am I just being blind?
Maybe it's something blatant that we're all missing, but it shouldn't really be possible to get that far through a puzzle without the setter giving us some inkling as to what's going on.
I'm with you, midazolam. Much tougher than this week's Listener, but a very much better puzzle, too. Ingeniously constructed, with a "pdm" worthy of it. 28dn was very clever, and is probably the key to completing the puzzle for those who are missing the final elusive step.
The same hint always given to Listener strugglers probably also bears repeating. "Read the preamble very carefully, and consider the significance of the title"