I obtained the theme from the extra letters in down clues, and it is one I revisit obsessively at regular intervals. I appreciate that this would be very hard if you were not familiar with the theme or had only encountered it in another form.
Having said that, I still found the puzzle very tough, and have not really thought about the final step at all. Moreover, I have an issue with 14 - the corrected letter seems to be wrong since it makes a mess of the first word of the instruction.