For the vinaigrette, place lemon juice, garlic, shallots and a pinch of salt and pepper in a medium bowl. Whisk to combine. Add olive oil in a steady stream while whisking to emulsify vinaigrette. Finish seasoning with additional salt and pepper to taste. For the couscous salad, first prepare cous cous by placing in a large bowl. In a saucepot, add water, harissa, herbs, salt and pepper and bring to a boil. Add spiced, boiling water to cous cous and cover with saran wrap or a lid for 20 minutes. Use 2 forks to fluff cous cous. Toss crab, sumac, peppers, chili, cous cous and ½ c lemon vinaigrette in a large bowl and season with salt and pepper to taste. In a separate large bowl, toss watercress and tomato with another ½ c vinaigrette and season to taste. Place ½ c cous cous in the center of four salad plates. Mound ¼ watercress salad mixture on top of each cous cous portion. Serve immediately.