So VERY frustrating: I have the theme, three hidden "linked items" with their small alterations, all 31 superfluous letters and the three "discrete groups" - two not groups at all of course - they spell out; and in fact the entire grid completed all except 3 cells (two crossing lights, one cell shared) in the bottom left, having no bearing on, and unaffected by, the theme and alterations. But I just cannot for the life of me see how or why to complete those.
SO: please can someone help me out of my misery? 38A, ?A?S, looks very much as if it wants to be DAIS (platform, I'm assuming, is the definition?); but I can't see why or how it works. And 35D is ?N?T, or if I'm right about DAIS, then ?NIT, but either way... stumped entirely. Which, after sailing through the rest, is really infuriating!