I don't ordinarily think of a D-minor scale as if it were a D-major scale with a lowered third, sixth, and seventh. I find it more useful to think of it as a cyclic permutation of F-major.

Thinking of a dorian scale in terms of modifying a major scale on the same tonic (by lowering the third and seventh) seems as unhelpful as thinking of a minor scale as a modification of a major scale on the same tonic. I suppose it might lead to insights that might not otherwise occur to one, but if I want to think of a dorian scale as related to other scales, I prefer to think of it as either (1) a cyclic permutation of the major scale with its tonic on the note a step below the dorian's tonic (X-dorian as a cyclic permutation of (X-1)-major); or (2) a minor scale on the same tonic with a raised 6th (X-dorian as X-minor with raised 6th.)

Likewise one can think of X-mixolydian as (1) a cyclic permutation of X+3 major or (2) X-major with a lowered 7th. I wouldn't ordinarily think of it as a modification of X-minor, though there's nothing illegal, immoral, or fattening about doing so.

If you look at my chording of Orientis Partibus above, you'll see that the melody-plus-chords has some characteristics of a work in C-major and some characteristics of a work in G-major. I find it easiest to think of it in the first instance as simply mixolydian, rather than "an air in C which closes on the dominant" or "an air in G with a lowered 7th."

T.