Impressed with the crossword, liked clues like 32A where the wordplay sprang to life when I deduced the missing letter, and found an unlikely word in Chambers.
Totally baffled by the endgame, not helped by not resolving
28D (which instrument? one fits wordplay better, the other holds out hope of every eventual answer being a real word, but final letter wouldn't be very helpful for bottom row)
36D (have a justifiable answer, but need to nail down...
...42A (one-letter abbreviation followed by 2-letter abbreviation? Can't find it with either initial letter from 28D)
I thought I'd be using thematic items from the hidden message to fill gaps, but this would be random rather than sequential. Hmmm.