In a 12-quart stockpot, bring wine and 3/4 cup water to a boil. Add garlic, shallots, potatoes, and 1/2 to 3/4 teaspoon red-pepper flakes, if using; cover and cook 8 minutes. Add corn, clams, and lemons; cover and cook until clams have opened, 10 to 12 minutes. Add shrimp in a single layer, cover, turn off heat, and let sit until shrimp are opaque throughout, about 3 minutes.
With tongs or a spider, transfer shellfish and vegetables to a serving platter, setting lemons aside; discard any unopened clams. Pour broth into a bowl and whisk in butter, parsley, and oregano. Scrape flesh from half the lemons into broth, discarding skins. Serve shellfish and vegetables with remaining lemons and broth for dipping.