In a large bowl, whisk together the 14 oz sweetened condensed milk and 1½ cups very cold water until smooth and fully combined.
Using an electric mixer on low speed, gradually add and blend one 3.4 oz box of vanilla instant pudding mix into the sweetened condensed milk mixture, mixing until smooth.
Cover the pudding base with plastic wrap and refrigerate until well chilled, at least 3 hours or overnight.
In a separate large bowl, beat the cold heavy cream with an electric mixer until stiff peaks form, about 3–4 minutes; increase speed as needed and then blend in 1½ teaspoons vanilla extract.
Gently fold the chilled pudding base into the whipped cream until combined and uniform in texture.
From the remaining box of dry vanilla pudding mix, add 5 tablespoons of the dry mix to the combined pudding and whipped cream, then beat briefly until the mixture is light and fluffy.
Slice the bananas. To assemble, place about one-third of the pudding mixture into a trifle bowl (or 9x13 pan), top with a layer of Nilla Wafers, and add one-third of the sliced bananas. Repeat layers once more and finish with the remaining pudding on top.
Make sure banana slices are fully covered by pudding to reduce browning, then cover the assembled dessert and chill at least 4 hours so the wafers soften.
Just before serving, garnish with reserved Nilla wafer crumbs and a few banana slices.