Whisk together 2 tbsp. lemon juice, 3 tbsp. oil, 3/4 tsp. salt, 1/2 tsp. pepper, the garlic, rosemary, and chopped thyme in a shallow dish. Add lamb and turn to coat. Chill, covered, 1 to 2 hours. Bring to room temperature 30 minutes before grilling.
Meanwhile, make jam: Put a small plate in freezer. Coarsely chop soft-ripe nectarines and put in a medium saucepan with 1 tbsp. lemon juice, 1 tbsp. water, the sugar, and lemon zest. Cook over medium heat, stirring and crushing with back of spoon, until fruit softens, 10 to 15 minutes. Boil gently, stirring, until a spoonful of jam put on the cold plate holds a soft shape, 10 minutes more. Stir in whole thyme leaves and more lemon juice to taste. Let cool.
Heat a grill to medium-high (450° to 500°). Cut firm-ripe nectarines in half and pit; brush with 1 tbsp. oil and sprinkle all over with salt and pepper to taste.
Grill chops (reserve marinade), turning once and basting with marinade, 8 to 10 minutes total for medium-rare. During last few minutes, grill nectarines, cut side down, until grill marks appear, 2 minutes. Cut fruit into wedges. Arrange meat and fruit on a platter and garnish with thyme sprigs. Serve with jam.