Start the potatoes. Place the partially peeled, cut potatoes in a large pot and cover with cold water by about 1 inch. Generously salt the water. Bring to a boil and cook for 15–20 minutes, or until completely fork-tender.
Cook the corn. While the potatoes boil, melt 3 tablespoons butter in a large skillet over medium heat. Add the fresh corn kernels and season with salt and pepper. Cook for 5–7 minutes, stirring occasionally, until tender but still crisp and juicy. Transfer to a bowl and tent loosely with foil.
Finish the mashed potatoes. Drain the potatoes thoroughly. In the empty pot, add 6 tablespoons butter and the half-and-half. Warm over medium-low heat just until the butter melts; do not boil. Whisk in the garlic powder. Return the potatoes to the pot and mash by hand to your desired consistency. Season with salt and pepper. Cover and keep warm.
Season the salmon. Pat the salmon completely dry. Drizzle lightly with olive oil and generously coat with Cajun seasoning.
Cook the salmon. Add about 2 tablespoons olive oil to the skillet over medium heat. Once hot, add the salmon presentation-side down (working in batches as needed). Cook undisturbed for about 3–4 minutes, until a deep golden crust develops. Flip and reduce the heat to medium-low. Continue cooking until the thickest portion reaches 145°F. Transfer to a plate and tent loosely with foil.
Clean the skillet as needed. If any Cajun seasoning remaining in the skillet has become very dark or burned, carefully wipe away the burned spices with a dry paper towel held with tongs.
Cook the shrimp, if using. Pat the shrimp dry, drizzle lightly with olive oil, and toss with 1 tablespoon Cajun seasoning. Add about 2 tablespoons olive oil to the skillet over medium heat. Arrange the shrimp in a single layer and cook for about 2 minutes per side, just until opaque and cooked through. Transfer to a plate and tent loosely with foil.
Start the cream sauce. Reduce the skillet to medium-low. Add 4 tablespoons butter and the minced garlic. Cook for about 30 seconds, stirring frequently, until fragrant.
Deglaze. Pour in the white wine and scrape up the flavorful browned bits from the skillet. Bring to a gentle simmer and cook for a few minutes, until reduced by approximately half.
Finish the sauce. Pour in the heavy cream and add 2 teaspoons Cajun seasoning. Bring to a gentle simmer and cook for about 5 minutes, stirring frequently, until the sauce begins to thicken. Reduce the heat to low and stir in the Parmesan until completely melted and smooth.
Add the lemon. Remove the skillet from the heat and stir in the fresh lemon juice. Taste and adjust with salt, pepper or additional Cajun seasoning as needed.
Assemble. To each plate, add mashed potatoes and buttered corn, then spoon Cajun Parmesan cream sauce onto the plate and top with salmon (and shrimp if using). Spoon additional sauce over the seafood and serve with lemon wedges.