Soak 12 long wooden skewers in water, at least 30 minutes. Meanwhile, toast the cumin seeds in a small skillet over medium heat, swirling the pan, until slightly golden, about 4 minutes. Transfer to a plate.
Combine the 3 chopped scallions, the jalapeno, ginger, garlic, 1 tablespoon vegetable oil, the brown sugar, 2 teaspoons soy sauce, the five-spice powder, 3/4 teaspoon salt and 1/2 teaspoon pepper in a food processor and pulse until a paste forms. Add the toasted cumin seeds; pulse once or twice to crack them slightly.
Lightly season the pork with salt and pepper; rub with about three-quarters of the cumin paste. Cover with plastic wrap and refrigerate at least 30 minutes and up to 1 hour.
Meanwhile, make the sauce: Combine 1 teaspoon of the remaining cumin paste, the remaining 1/4 cup soy sauce, the vinegar, sesame oil and 1 tablespoon water in a small bowl. Stir in the sliced scallion and set aside.
Preheat a grill to medium. Combine the remaining cumin paste and 2 tablespoons vegetable oil in a small bowl; set aside for basting. Generously brush the grill grates with vegetable oil. Thread 3 or 4 pieces of pork on each skewer and brush with vegetable oil. Grill the kebabs, turning, until marked all over, 6 to 9 minutes, basting with the cumin-oil mixture during the last minute. Let rest 5 minutes before serving. Drizzle with the sauce.