Hilary, the simplest answer to your question is that you're correct. Just realize that the minor keys available in the 'D' and 'G' fingerings you've described above won't always lie in a good playing range. For instance, since the relative minor to D is Bm, the root note of the scale would, of course, be 'B.' Thus, for any Bm tune which resolved to an ending B, you'd be playing most of the tune in the second octave of the whistle and possibly even the third. Could get pretty shrill.