I think what you are looking for is called Ribbon Salad, from "Joys of Jello Gelatin Dessert", a booklet about 5"X7", published as near as I can figure about 1962. In this recipe the marshmallows, cream cheese, mayo, whipped cream, and crushed pineapple are in the middle layer of lemon jello. No double boiler mentioned, but that could have been your Mom's own version, like the marshmallow creme in fudge. I wonder if the marshmallows would melt in the double boiler and give it a smoother texture. There are many postings of this recipe on the Internet, just google "Joys of Jello Ribbon Salad".
In that Taste of Home recipe, the poster indicates that she substitutes sugar free, fat free products.