Combine 5 cups cold water, the lentils, garlic and 2 teaspoons salt in a medium saucepan. Bring to a boil, and then reduce to a simmer. Simmer until the lentils are tender but still hold their shape, 20 to 25 minutes, then drain. Remove the garlic halves and mash them with the flat side of a knife. Place the mashed garlic in a large bowl. Whisk in 2 tablespoons of the oil, 1/4 teaspoon of the cumin, the lemon juice, a few grinds of pepper and 1/2 teaspoon salt. Add the drained lentils and toss to coat.
Heat a grill pan set over medium-high heat. Place the shrimp in a medium bowl and the tomatoes and squash in another bowl. To each bowl, add 1 tablespoon of the olive oil, 1/4 teaspoon cumin, 1/4 teaspoon coriander, 1/4 teaspoon paprika, 1/4 teaspoon salt and a pinch of pepper. Toss to coat. Thread 2 pieces of shrimp, 2 tomatoes and 2 pieces of squash onto each skewer. Rest the kebabs at room temperature for 15 minutes.
Grill the skewers until the shrimp are firm and opaque and the squash are firm-tender, 1 1/2 to 2 minutes per side. Sprinkle lightly with salt.
Fold the dill, parsley and scallions into the lentil salad, then divide among four plates. Top each salad with 2 skewers. Serve with lemon wedges on the side.
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