20a is not thematic and is an anagram of the first three words (minus a letter, of course). We've already had the solution to 21d very early on.
I am still puzzled by 6a & 7d. I have solutions, but the wordplay for 6a seems to lead to the Latin word for "nothing" and doesn't seem to apply to any other clue. I assume the word play in 12 applies to 6a but I can't parse it and I can't find any wordplay that applies to 7d.