In a saucepan, combine the sugar, pineapple juice, lemon juice, and salt over medium heat, stirring constantly, until thick. Remove from heat and let cool. Whip cream and fold it into the cooled syrup mixture. Stir in marshmallows, grapes, pineapple, apples, celery and nuts. Spread mixture into an 8 by 8-inch pan and freeze until frozen throughout, at least 2 hours. Remove the salad from the freezer 5 minutes before serving. Cut into 8 squares. Garnish each serving with a cherry and a sprig of fresh mint.