The not-official thematic item really messed me about because I spent ages looking for it in the grid (as it would have made it in) and it just wasn't. Finally I Google'd it and learned something :)
The 'internal letters' bit of the preamble could have been better worded (or perhaps 'more helpfully' is a more accurate way of saying it - I suspect Yorick deliberately chose something a bit opaque). Took me ages for the penny to drop, and I dread to think how many rabbit holes I went down. I could see that some of the clues had a slightly less natural reading, but once I worked out what was going on, I was surprised; I was expecting something that put more of a constraint on the setter given the surface readings. Go figure.
Very satisfying to finish - if that's it for the year, all that's left to do is wait and see how many silly mistakes I've made, as I've submitted all puzzles this year (which may be a first!).