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Whisk together lemon juice, sugar, chile, garlic, fish sauce (to taste), and 1/2 tsp salt. Let stand while preparing remaining ingredients.
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Cut off top inch of pummelo and scoop out flesh, leaving a shell. Pull off interior pith and remove segments, then remove segment membranes and any small seeds, separating pulp. (Fruit may remain in small clumps.)
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Cook pork belly in a small saucepan of boiling salted water (1 1/2 tsp salt for 1 qt water) until cooked through, about 10 minutes. Drain and cool slightly, then cut into 1/4-inch-thick sticks.
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Cook shrimp in boiling salted water (1 1/2 tsp salt for 1 qt water) until just cooked through, about 3 minutes. Drain and cool slightly, then peel shrimp.
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Heat oil in a small heavy skillet over medium heat until it shimmers. Fry shallot, stirring frequently, until golden brown. Transfer to paper towels to drain.
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Toss together pummelo pulp, pork belly, shrimp, knotgrass, sesame seeds, and lemon dressing to taste. Season with salt and put in pummelo shell. Top with fried shallots.
