Prepare the compote: Preheat the oven to 450 degrees F. Core the tomatoes and halve crosswise, then squeeze out the seeds. Put the tomatoes cut-side up on a baking sheet. Drizzle with 3 tablespoons olive oil; sprinkle with 1 teaspoon salt, and pepper to taste. Roast until shriveled, about 40 minutes. Cool slightly, then slip off the skins. Cut into chunks.
Toast the coriander and cumin seeds in a skillet over medium heat, about 2 minutes. Remove from the skillet and grind with a spice grinder or chop with a knife. Heat the remaining 2 tablespoons olive oil, the garlic and ginger in the skillet over medium heat and cook until beginning to brown, about 3 minutes. Add the tomatoes, ground coriander and cumin, and salt and pepper to taste.
Prepare the crabs: Heat 1/8 inch vegetable oil in a large nonstick skillet over medium-high heat. Put the polenta in a baking dish; add 2 teaspoons salt and 1 teaspoon pepper. Put the milk in a bowl. Dip the crabs in the milk and shake off the excess, then dredge in the polenta and shake off the excess.
Working in batches, fry the crabs, upside-down first, 3 to 4 minutes per side. Transfer to a paper towellined plate. Wipe out the skillet and add fresh oil between batches.
Reheat the compote and stir in the herbs. Serve the crabs with the compote and lime wedges.