Bring a large pot or wide skillet of water to a boil. Off the heat, add the noodles. Briefly swish them around to separate them, then let sit, stirring occasionally to prevent sticking, until the noodles are soft and pliable but still chewy to the bite, 5 to 10 minutes. Drain and rinse well with cold water. Set aside.

In a small bowl, beat the eggs with a pinch of salt. Set aside.

Make the sauce: In a medium bowl, whisk together the water, fish sauce, soy sauce, vinegar, ketchup, sugar, and red pepper flakes. Set aside.

Heat 2 tablespoons of the oil in a large nonstick pan or wok over medium-high heat. Add the shrimp and season with ⅛ teaspoon salt. Cook, stirring often, for two minutes. Add the garlic and light green scallions and cook, stirring constantly, until softened and the shrimp are cooked through, about 1 minute more. Transfer the shrimp, garlic, and scallions to a large plate, using a rubber spatula to scrape the pan clean.

Add 1 teaspoon of oil to the hot pan. Add the eggs and scramble until cooked through, 1 to 2 minutes. Add to the plate with the shrimp.

Add 2 tablespoons of the oil to the hot pan. Add the drained noodles to the pan, along with the sauce mixture. Cook, tossing the noodles gently so as not to break them, until the liquid is absorbed by the noodles and the noodles are cooked through, a few minutes. If the noodles are still firm to the bite when the sauce is absorbed, add a few tablespoons of water and continue cooking. Add the contents of the plate and the dark green scallions to the noodles and toss gently to combine, until everything is warmed through. Taste and adjust seasoning if necessary. Transfer to a serving platter and top with peanuts, bean sprouts, and cilantro (if using). Serve with lime wedges, if desired.