Full grid but baffled over 'yes-no' question. Reading l to r, top to bottom 7, 9 and 15 give message 5, 5, 5, 3, 2, 4, 4 which make no sense to me in terms of the instructions.
No problem with answer 20 which sort of makes sense both with and without the unchecked letters.
Any nudges gratefully received.