The first sentence of the preamble is, I think, deficient, which slightly spoiled an otherwise excellent puzzle. Where an across entry would require two letters in a cell, there are in most cases several cells that it could be, and to say that “only the letter checked” must be entered really doesn’t resolve much. Once the penny dropped, I simply used the words it was now clear we were heading for to help me determine which cells would be the ones in which I would have written two letters, and I replaced the ones checked with the others to show the outcome. The resulting across before and after were nonwords. I was also a bit disappointed that one of them was in the singular, when it is plainly plural in the source. Still, lots of fun otherwise and I got there in the end. I am amused to think that a strict reading of the instruction means anyone who didn’t send their puzzle in has not in fact fully completed it 🤣
A few clues were fiendish, a few very clever, and some delightful. I haven’t quite cracked the wordplay for the second part of 18, “… master, replacing head’s debut on …”, if anyone can nudge me across the finishing line with that please…