Thanks, Prospero.
I now have the grid completed, the 15,30,7 phrase, the extra-word instruction, that instruction found and "understood," and all 6,7 words matched with either 15 or 30. I am not sure whether the fact I can write and notate music hinders me, but I have little idea what the devil is going on, except that the whole crossword is a ***** and one considers every **** rows.
But I am again not asking for help, as I am giving up for the second consecutive month in many years (thanks to the correspondents on here). I may tender my resignation from Mensa!