As I understand it, Casanova’s query is about the 15 letters that spell out the name of the artwork. These can be found in the grid, not in clue order but reading row by row as stated in the preamble.
Thanks as ever to the members of the forum who have provided much-needed help. I spent several hours battling with the clues, producing just four four-letter answers, and then gave up. Last night, however, I took advantage of the hints and answers here, and it was amazing how quickly it all fell into place.