In a bowl stir together all dressing ingredients until sugar is dissolved. Chill dressing, covered, 1 hour. Shell and devein shrimp. In a large saucepan of salted boiling water cook shrimp with lemon 2 minutes, or until just cooked through. In a colander drain shrimp and rinse under cold water to stop cooking. Halve shrimp lengthwise.
Halve cucumber lengthwise and seed. Cut cucumber halves and scallions crosswise into 2 1/2 inch pieces and cut pieces lenghtwise into julienne strips.
In a bowl soak noodles in hot water to cover 15 minutes to soften. While noodles are soaking, in a 6-quart kettle 31/2 quarts salted water to a boil. Drain noodles in colander and cook in boiling water 45 seconds, or until just tender. Drain noodles in colander and rinse under cold water to stop cooking. Drain noodles well and with scissors cut into 4-inch lengths.
Divide noodles, shrimp, vegetables, and dressing among 4 bowls and sprinkle with herbs and peanuts. To serve, pull herbs from sprigs and toss with noodles.