Nicely put, Jack. I don't think it's giving too much away at this stage to say that NO specialist knowledge is required to arrive at the final grid, provided one has solved all the non-normal clues and read the relevant Wiki (or equivalent) article relating to (c). It helps if, as I did, you guess the identity of 18 12 quite early on so that you get a pointer as to which cells have their contents replaced.
The three normal clues are arguably more of a hindrance than a help, in that at least two of them are so convoluted that they are almost impossible to cold-solve (they were for me, anyway). Unless you are familiar with the answers, and do have specialist knowledge, the grid entries offer no help at all. I only know one of them and was able to use the relevant grid entry to confirm it; the other two I reverse engineered after searching for lists of possibles whose (a) is compatible with (c). But it isn't actually necessary to solve them at all, which will rile solvers who spent ages trying to make sense of the grid entries only to find they needn't have bothered!
I cited SmellyHarry's post because yes, the resulting grid is a mess. After Ifor's masterly puzzle last week, in which different possible entries for the top row all led to real words, this is a disappointment.
But like you I see this as a bit of seasonal
jeu d’esprit , and it's a shame that it backfired for some due to the puzzle appearing to be far more complicated and specialist than it actually is.
Anyway, Happy New Year to all posters and, of course, the admin team.