Add 1 package of raspberries and half of the strawberries to saucepan with 1 c of sugar and lemon juice. Bring to a boil, stirring often; immediately reduce heat and simmer 10 min, until syrupy. Remove from heat. Add remaining raspberries, strawberries and blueberries to mixture, along with liqueur, if using. Chill completely, 1 hour.
With an electric mixer, whip cream with 3 T of sugar and vanilla until stiff peaks form. Refrigerate until ready to use. To serve, either use individual glasses or one large dish and layer in this order: bottom quarter of dish with berry mixture, then half of the whipped cream and meringue pieces (dish should be half full), add another layer of berries and finish with more whipped cream and meringue, topping with a final drizzle of berry sauce. Garnish with mint sprigs or, for a twist, a sprinkle of shaved chocolate. Serve immediately.