Preheat the grill for direct grilling, high heat.
Toss the potatoes with the canola oil, 2 tablespoons chopped dill, 1 teaspoon smoked paprika and sprinkle with salt and pepper. Transfer to a grill basket and grill, tossing occasionally, until golden brown and cooked through, about 10 minutes.
Combine the mayonnaise, cider vinegar, horseradish, mustard, honey, and salt and pepper to taste, and whisk to combine. Slowly whisk in the olive oil.
Drizzle some of the dressing over the bottom of a large serving platter or shallow bowl. Pour the warm potatoes on top. Scatter the Smoked Trout over the potatoes, then the watercress and dill fronds. Dust with more smoked paprika and drizzle a generous amount of dressing over all.
Combine the salt, sugar and 4 cups water in a 4-quart container and stir until the salt has dissolved, 1 to 2 minutes. Add the trout, making sure they are submerged. Cover and refrigerate for at least 1 hour and up to 3 hours.
Remove the trout from the brine, rinse thoroughly with cold water and pat dry. Place the trout skin-side down onto a rack set in a half sheet pan. Dry in the refrigerator until the skin becomes shiny and somewhat tacky to the touch, 21 to 24 hours.
Prepare the smoker using the alder or applewood chips so that it maintains a temperature of 150 to 160 degrees F.
Place the trout onto the smoking racks, skin-side down, separating them by at least 1/4-inch and place into the smoker. Adjust the heat as needed and cook until the fish has cooked through, darkened in color around the edges and has the desired level of smoke flavor, about 20 minutes. Remove from the smoker and cool. Remove the flesh from the skin and gently break into big chunks.