On re-visiting this, I am still puzzled about the last three:
16A Busy route going across yacht (4). I have M-X-;
18D Be permanently present at the bottom of a shaft, drinking ditchwater primarily (7) . I have I---E-L. Idoneal? But why?
22A Revolutionary musical director's opening a case (6) --T-V- . Octave?
All help gratefully received!