For dressing: Whisk mayonnaise, sour cream, pickle juice, Dijon, and sugar together in a small bowl. Crush cumin seeds and celery seeds together in a mortar & pestle, or give them a quick whiz in a spice grinder, just enough so they are coarsely crushed. Add crushed seeds, dill weed, and Mt. Elbert to mayo mixture and stir to combine. Season with salt & pepper to taste. Set aside so flavors can meld while salad is prepared.
For salad: In a large pot, cover potatoes with cold water by about 1 inch. Add salt and bring to a boil. Boil until largest pieces are easily pierced with a knife, 10 to 15 min. Drain potatoes into a colander and run cold water over them for a minute to stop the cooking. Once cool, cut again into bite-size pieces and place in a large bowl. Add celery, scallion, and pickles. Fold in dressing to taste. Season with more salt & pepper to taste. Cover and chill for at least an hour before serving. Garnish with additional cumin seeds if desired.