I don't know when the two-letter non-gendered pronoun was invented - not long ago, I think - but I was aware of it so had no problem with 15A. Nevetheless, I was surprised that it is not in my latest edition of Chambers.
I managed to solve most of the puzzle without help, and only referred to the Forum for some of the parsing. However, despite chrise's comment, I still do not understand the parsing for 1,4. Please explain.