Devein the shrimp, but leave the shell on. Using a small sharp knife, cut a slit along the back of the shrimp through the shell and remove the vein. Divide into two large mixing bowls, one for the jerk shrimp and the other for the garlic and herb shrimp.
In one of the bowls, toss the shrimp with 2 tablespoons of the olive oil, salt, pepper, thyme, rosemary, jerk seasoning and lemon juice. Stuff any extra herbs or seasoning left in the bottom of the bowl into the slit of the shrimp.
Skewer the shrimp and place over medium-low direct heat on the grill. Cook for about 10 minutes total, flipping halfway through. Remove from grill and garnish with slices of fresh chiles.
In a separate bowl, toss the remaining shrimp with 2 tablespoons olive oil, grated garlic, salt, pepper and mixed herbs.
Grill over medium-low direct heat for about 10 minutes, flipping halfway through. Remove the shrimp and place on a platter, then drizzle with half of the melted butter.
Serve the shrimp with blistered shishito peppers and a bowl of the remaining melted butter for dipping.