Make the mashed potatoes: Place potatoes in a 4-qt. saucepan and cover with water by 1″; bring to a boil over high heat, and cook until potatoes are tender, about 15 minutes. Drain potatoes and pass through a ricer or food mill set over a bowl; set aside. Meanwhile bring 1 1⁄2 cups cream and 12 tbsp. butter to a boil in a 1-qt. saucepan; pour over potatoes and whisk until smooth. Season with salt and pepper, and keep warm until ready to serve.
Make the meatballs: Place 1⁄2 cup cream and bread in a small bowl; let sit until soft, about 5 minutes. Transfer to a large bowl along with pork, beef, allspice, egg, and onion, season with salt and pepper, and mix until evenly combined. Shape mixture into about thirty 1-oz. balls, about 1″ in diameter. Heat remaining butter in a 12″ skillet over medium heat; working in batches, add meatballs and cook, turning as needed, until browned all over and cooked through, about 12 minutes. Using a slotted spoon, transfer meatballs to a plate and set aside.
Return skillet to medium-high heat. Add flour, and cook, stirring, until smooth and light brown, about 4 minutes. Whisk in beef stock until smooth, and then bring to a boil; stir in remaining cream and return meatballs to gravy. Reduce heat to medium, and cook, stirring gently, until meatballs are warmed through, about 3 minutes; season with salt and pepper. Serve meatballs and gravy over mashed potatoes and garnish with a generous dollop of lingonberry preserves.