1. Finely grate the zest from 4 of the lemons, then pulse in a mini food processor with 3 tablespoons salt. Squeeze the juice from 2 of the zested lemons into a large bowl; add the lemongrass, olive oil, Sriracha, garlic, fish sauce, brown sugar and 1 teaspoon of the lemon salt. Thinly slice the 2 remaining lemons and add to the bowl.
2. Using kitchen shears, cut through the shrimp shells along the outer curve, leaving the shells on. Remove the veins and rinse the shrimp. Add to the bowl with the lemongrass marinade and toss; cover and refrigerate for 1 hour. Meanwhile, soak 12 to 15 wooden skewers in water.
3. Preheat a grill to medium high. Thread the shrimp onto the skewers. Brush the grill with vegetable oil, then add the lemon slices and grill until they begin to char, turning once, about 2 minutes; transfer to a platter. Stir the mint into the remaining marinade. Add the shrimp skewers to the grill and cook, brushing with the marinade-mint mixture, until the shells begin to char, 2 to 3 minutes per side. Transfer to the platter and sprinkle with the lemon salt to taste.