1. Trim off root and pull coarse outer leaves from lemon grass and discard. Pound lemon grass to bruise slightly. Cut stalks in half lengthwise, then crosswise into about 3-inch pieces.
2. In a 2- to 3-quart pan over high heat, combine lemon grass and clam juice and bring to boiling. Reduce heat, cover, and simmer 10 minutes.
3. Pour clam broth through a fine strainer into a bowl. Return broth to pan and discard lemon grass.
4. Stir tomatoes, lime peel, lime juice, and jalapeño into clam juice; set aside.
5. Gently mix shrimp and scallops with oil, then mix with 1 tablespoon each of minced mint and basil. Thread seafood horizontally onto thin metal skewers.
6. Lightly oil a barbecue grill over a solid bed of hot coals or gas grill on high heat (you can hold your hand at grill level only 2 to 3 seconds); lay corn on grill. Close lid on gas grill. Turn corn often until it is speckled black, about 12 minutes.
7. About 5 minutes before corn is done, lay skewered seafood on grill. Cook, turning once with a wide spatula, until shrimp are opaque but still moist-looking in center of thickest part (cut to test), 3 to 4 minutes.
8. Cut kernels from corn, and pull seafood from skewers; keep warm.
9. Over high heat, bring broth mixture to simmering. Stir in remaining minced mint and basil. Ladle broth into wide bowls. Mound corn, seafood, and fennel in each bowl. Sprinkle with chives, and season to taste with fish sauce.