Combine flour, 1 tsp. salt, 1 tsp. pepper, and the cayenne in a large bowl. Pat crab pieces dry with paper towels and toss (in batches) with flour mixture. Remove crab and shake off excess flour. Set aside.
In a wok or large pot, heat 3 in. oil to 375°. Lay out paper towels for draining crab and garlic. Fry crab in batches (do not crowd wok) until golden, about 5 minutes per batch. Drain on paper towels.
Using the same hot oil, fry garlic until golden brown, 2 to 3 minutes. Remove with a slotted spoon and drain on paper towels. Set garlic aside; cool and discard oil.
Bring a large pot of water to a boil. Add 1 tbsp. of the salt and the spaghettini. Cook until tender to the bite, 5 to 10 minutes. Drain, transfer to a serving bowl, and toss with butter and half of the fried garlic. Cover and put in a warm place.
Heat a wok or pot large enough to hold all the crab over high heat. Add remaining 3 tbsp. oil, dried chiles, and ginger. Cook, stirring constantly, until fragrant, about 30 seconds. Add green onions, serrano chiles, and remaining 1/2 tsp. salt. Cook, stirring, until onions wilt, about 1 minute. Add sake and cook, stirring, until sake is reduced by about half. Stir in crab and cover. Cook until crab is heated through, about 3 minutes.
Remove lid and cook, stirring, until any liquid evaporates. Stir in basil, mint, cilantro, and remaining 1/2 tsp. pepper. Cook, stirring, until herbs have wilted. Stir in remaining fried garlic. Transfer crab to a warm platter and serve hot, with garlic noodles.
Note: Nutritional analysis is per serving.