Lumen, in 19d - Yes, Sienna adjusted. It's an anagram with he longest word supplying 1 letter. Def. is the last 4 words.
1a. def. is 'triforia'. Adjustment to 'acted'.
4 - last word is altered to Scottish word. Def is last 3 words. Answer is another Scottish word.
Please excuse my ignorance, but I'm fairly new to this forum: what does GWIT stand for?