Quisling (#50, 51, 52)
As you say, in 25d, the clue WITHOUT a correction can be read as wordplay leading to a four-letter word + a single letter, giving the answer but without a definition.
However, WITH the correction, the definition can lead to the same four-letter word since Wikipedia lists it as a Spanish SURNAME; the addition of the single letter gives a FORENAME, so this clue is a legit &lit.
Thanks to everyone who posted hints to obscure wordplay in the down clues which gave me peace of mind. I was still flummoxed by the wordplay in 14a which no-one here queried but I just realised what 'unacceptable today' meant and all became as crystal.
I've just returned from my fairly regular early-Monday-morning trip to the postbox round the corner — to catch the first collection — after the street lights have timed out. The light from the moon was very welcome.
Thanks to Quinapalus for a fairly straightforward but enjoyable workout. I did, however, make the endgame far more time-consuming than it should have been by skimming the instruction in the preamble and not taking in the fact that the initial letters of the additional thematic items appear IN ORDER... D'oh!