I am ambivalent towards this puzzle: I thought the main solve was unnecessarily difficult, but the endgame was terrific.
I enjoy carte blanche, but am a below-average cold solver so find clashes tricky. Those clashes, combined with the esoteric entry-filling mechanism in some clues, sapped a lot of the fun out of the carte blanche -- and the clues themselves were on the harder side for me, even before the possibility of (multiple) unindicated letters. For the first time ever I skimmed the answer thread here to look for hints on some of the difficult clues. (I do wish the Chambers app had a reverse lookup feature where I could enter "Spenser" or "Scot"; the CD-ROM version seems very hard to get hold of.)
The preamble made sense in the end -- a lot of its trickiness was in the complexity of what it described. But I must say "24 letters are ignored in their clues' wordplay" is very easy to misunderstand as the opposite of what it actually means; a strict parsing of the preamble suggests it's fair, but it was yet another obstacle to me.
The endgame itself though was delightful, and yet another astonishingly clever piece of construction. Once I'd hit an (abnormally high, I think) critical mass of solved clues, the real fun began -- and this was difficult too, but in a way I thoroughly enjoyed. And it was nice to have thematic material I recognised.
So I'm here to both vent my frustration over something that seemed overly punishing (I think I resent it more because I recently calculated each Listener costs me £6 thanks to the Times' one-size-fits-all digital subscription, which has left me grumbling) -- and also to express much admiration for a superb endgame.