I completed all of this puzzle in a couple of days, apart from 7d which left me flummoxed, so I put it aside until today. On returning to it -- having turned into a septuagenarian in the interim -- I had to remind myself of the theme... and thus all became as crystal!
As others here have found, there are 16 clues (excluding 19a) were a letter or letters are ignored, rather than 15 as stated in the standfirst (new word to me). I'm putting this down to the curse of The Grauniad!
I've asked this before, but never received a response: where can I find the solution to Genius puzzles AND the name of the lucky winner of the £100 prize? If both are in the paper, that seems rather strange for what is billed as an online competition. Is there somewhere else in the Guardian website where this information can be accessed?