Throck was correct in stating that the additions are 1, 2, 3, 4 and 5 letters respectively (and that the added 3 make 24d slightly dodgy as a clue). There are so many possibilities for the additions that I confess I just Googled the first sentence of the preamble, got the theme, and worked backwards to get the additions. After too long of a grid stare looking for something profound, the highlighting was simple and perhaps a bit of a letdown,