Make the pickling liquid: Combine white vinegar, sugar, 2 cloves crushed garlic, crushed Thai chiles, and crushed lemongrass in a medium saucepan. Bring to a boil over medium heat. Remove from heat, transfer to a bowl, and let cool.
Slice down either side of the pit of the mango, forming two halves. Using a mandoline, thinly slice mango halves lengthwise. Add mango to pickling liquid, and pickle for about 30 minutes.
Make dressing: In a medium bowl, combine chile sauce, 1 tablespoon reserved grapefruit juice, and 1 tablespoon pickling liquid. Whisk in olive oil to emulsify. Season with salt and pepper. If too hot, add more pickling liquid.
In a large bowl, mix endive, watercress, shallot, sliced lemongrass, sliced garlic, finger chile, mint, cilantro, and basil leaves. Add half of the grapefruit pieces; toss. Add dressing to salad, and toss. Drain the pickled mango; set aside.
Divide salad equally among 4 plates. Divide the pickled mango equally, and place on top and around greens. Garnish each plate with remaining grapefruit pieces, 1 tablespoon shredded coconut, Thai pickled lemon, and remaining cilantro sprigs. Serve immediately.
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