Pour quarter cup cold water into a heatproof glass measuring cup, sprinkle in the gelatin and let soften 2-3 minutes. Set the cup in a skillet of simmering water and stir the gelatin over low heat until it dissolves. Remove skillet from heat, but leave the cup in to keep the gelatin warm. With a whisk or electric beater, beat the yolks until well blended. Slowly add quarter cup of the regular sugar and continue beating until the yolks are thick enough to fall in a ribbon when the beater is lifted from the bowl. Beating constantly, pour in the boiling water in a thin stream then pour the mixture into a 1.5 to 2 litre enemeled or stainless steel saucepan.
Stir over low heat until it thickens into a custard heavy enough to coat the spoon. Do not let custard boil, or it may curdle. Remove the pan from the heat and stir in the dissolved gelatin, then strain the custard through a fine sieve set over a deep bowl and add 3 tablespoons of the rum and the lemon peel. Let the custard cool to room temperature, stirring occassionally to prevent it from setting. In a seperate bowl, beat egg whites and salt with a clean whisk or beater until they are frothy. Sprinkle in the remaining regular sugar and continue beating until the whites form peaks. Stir about quarter of the whites into the custard, then pour it over the remaining egg whites and fold together with a spatula. Fold in 1.25 cup of the nuts, pour the chiffon mixture into the pie shell and smooth the top with the spatula. Refigerate until served. Just before serving, beat the heavy cream with a wire whisk or mixer until it thickens. Add the superfine sugar and the remaining 2 tablespoons rum, continue beating until the cream is stiff. With a spatula, spread the cream over the pie, sprinkle remaining nuts over the top.