1. In a food processor, combine garlic, sliced shallots, macadamia nuts, galangal, and turmeric. Whirl mixture to a paste, scraping container sides as needed. (Or chop ingredients with a knife, then whirl in a blender to a paste.)
2. In a 5- to 6-quart pan over high heat, stir the paste and the oil until mixture barely begins to brown, about 3 minutes. Stir in broth, cover, and bring to a boil.
3. Meanwhile, trim stem end and any tough leaves off lemon grass and pull off coarse outer layer. Crush stalk with the back of a knife, then cut into 3-inch pieces. Add to broth.
4. Add chicken breasts to broth. Cover, bring to a boil, then reduce heat to low and simmer until the breasts are no longer pink in center of thickest part (cut to test), 15 to 20 minutes. Lift out chicken and let cool at least 10 minutes. Skim fat from broth. (Or if making ahead, cover and chill the broth and the chicken separately. Lift off and discard fat from the chilled broth.)
5. In a bowl, pour 5 cups hot water over bean thread noodles. Let stand until the noodles are tender to bite, about 5 minutes; drain. If desired, snip through noodles with scissors to make shorter strands.
6. Season broth with salt and pepper to taste. Return to a boil over high heat.
7. Tear the chicken into shreds. In individual small bowls, mound shredded chicken, bean thread noodles, cabbage, bean sprouts, green onions, tomatoes, cilantro, egg wedges, potato chips, fried shallots, lime wedges, and chili sambal. Pour hot broth into a tureen or pitcher.
8. Let guests place desired portions of the chicken, noodles, cabbage, bean sprouts, green onions, tomatoes, cilantro, and eggs in wide bowls. Ladle or pour hot broth into bowls. Sprinkle with potato chips and fried shallots. Add juice from lime wedges, chili sambal, and more salt and pepper to taste.