Went on 17th August to Ben Macdui, as planned. As you suspected, no sign of any dotterel, although having approached from Braemar I didn't have much time to explore the summit plateau in any case, because it's such a long walk in. But I did see two snow bunting shivering on the top in a hail storm, a golden eagle, ptarmigan, red grouse, a redstart, crossbill, arctic hare, four roe deer and great views, so still quite a succesful day.
Will try again another year earlier in the season.