Noodles: Fill a bowl with lukewarm water. Pull apart noodles and roughly cut into 6-inch bunches. Place the noodles in the water so they are fully submerged and soak for 20 minutes. (Remaining noodles can be stored in a sealed plastic bag in the refrigerator for up to 1 week.) If you're using dried rice noodles, soak in water for 30 minutes, or until very pliable.
While noodles are soaking, prep the other ingredients: Slice tofu into ¼-inch thick slices, then slice into small chunks, about ¾-inch long. Chop 6 garlic chives into 1½-inch pieces; you should have about ¼ cup. Soak salted radish in lukewarm water, 15 minutes; then drain and soak for another 5 minutes. Drain and roughly chop. Cut the Key lime into quarters. In a small bowl, combine tamarind water, palm sugar simple syrup, and fish sauce. (To make tamarind water from tamarind pulp: boil 1 cup water and 2 tablespoons tamarind pulp; let cool for 30 minutes; strain.)
Pad thai: Heat wok over medium-high heat and add oil. Crack the egg into a small bowl. When wok is smoking hot, add the egg and let it cook undisturbed. Immediately add shrimp, followed by tofu, dried shrimp, and salted radish. Use metal spatula to break up egg yolk and turn everything over to sear. Remove noodles from water, shake off excess water, and add to the wok. Toss vigorously, then add the sauce; stir-fry for 1 minute, allowing the sauce to soak into the noodles. Add the bean sprouts and most of the chopped garlic chives; stir to combine, 20 seconds.
Serve: Transfer pad thai to a serving platter. Garnish with crushed peanuts, remaining chopped garlic chives, chile powder, and a pinch of sugar. Serve immediately, with Key lime wedges and 4 garlic chives on the side.