Place racks in upper and lower thirds of oven; preheat oven to 375 degrees. With a fork, pierce potatoes in several places. Place on a rimmed baking sheet; bake on lower rack until tender, 45 to 55 minutes.
Meanwhile, in a small bowl, combine bread and 1/4 cup milk. In a large bowl, combine beef, onion, garlic, parsley, egg, thyme, oregano, 3/4 cup marinara sauce, 2 teaspoons coarse salt, 1/4 teaspoon pepper, and bread mixture. With a fork or your hands, mix very gently until just combined.
Transfer mixture to a rimmed baking sheet, and gently shape into a 9-by-5-inch loaf, about 2 1/2 inches thick. Bake on upper rack 30 minutes; then spread 1/4 cup marinara sauce over meatloaf, and continue to bake until an instant-read thermometer inserted in center registers 160 degrees. 25 to 30 minutes more. Let meatloaf rest on baking sheet, loosely covered with aluminum foil, 10 minutes.
While meatloaf rests, in a medium saucepan, bring butter and remaining 1 cup milk just to a boil. Add cooked potatoes and mash with a potato masher. Season with salt and pepper.
Remove meatloaf from baking sheet. Slice; serve with mashed potatoes and remaining cup marinara sauce (heated, if desired).