To prepare crab cakes, place a small sieve or colander in a 2-quart glass measure or medium bowl. Line colander with 4 layers of cheesecloth, allowing cheesecloth to extend over sides. Spoon yogurt into colander (A). Cover loosely with plastic wrap; refrigerate 12 hours. Spoon yogurt cheese into a bowl; discard liquid.
Combine yogurt cheese, onion, and next 10 ingredients (through crackers). Add crab; stir just until blended. Divide mixture into 12 equal portions, shaping each into a 1/2-inch-thick patty. Heat 1 teaspoon oil in a large nonstick skillet over medium-high heat. Add 6 patties; cook 3 minutes per side or until lightly browned. Remove from pan; cover and keep warm. Repeat procedure with remaining 1 teaspoon oil and 6 patties.
To prepare salad, combine vinegar and next 6 ingredients (through red pepper), stirring with a whisk. Place greens and watercress in a large bowl. Drizzle with vinaigrette; toss well to coat. Arrange about 1 cup salad on each of 4 plates; top each serving with 3 crab cakes.
Note: You can also use a quick-drain method for the yogurt. Spread yogurt onto several layers of paper towels (B), and cover with more paper towels. Let stand five minutes, and scrape into a bowl. Quick-drained yogurt isn't as thick as yogurt that's drained overnight, so start with a little bit less to ensure the crab mixture won't become too watery.