Combine first 8 ingredients and 3 garlic cloves in a small bowl. Cut tofu crosswise into 6 slices. Pat each square dry with paper towels. Place tofu slices on a jelly-roll pan. Brush both sides of tofu slices with lemon juice mixture; reserve remaining juice mixture. Let tofu stand 1 hour.
Place tofu slices on grill rack coated with cooking spray; grill for 3 minutes on each side. Brush tofu with the reserved juice mixture.
Combine remaining 1 minced garlic clove, chopped olives, sour cream, and mayonnaise in a small bowl; stir well. Spread about 1 1/2 tablespoons mayonnaise mixture over bottom half of each hamburger bun; top each serving with 1 tofu slice, 1 tomato slice, about 2 tablespoons watercress, and top half of bun.