Preheat oven to broil with rack about 5 inches from heat. Line a large rimmed baking sheet with foil. Toss together bananas and 1 ½ tablespoons oil on baking sheet. Broil until browned and softened, 12 to 18 minutes. Transfer bananas and any juices to a large bowl. Mash bananas using a fork to yield 1 ½ cups; discard or reserve any remaining banana for another use. Let cool slightly, about 10 minutes, stirring often.

Reduce oven temperature to 350°F with rack in center position. Add eggs, pineapple, granulated sugar, brown sugar, vanilla, and remaining 1 cup oil to bowl with banana; whisk until well combined. Stir in flour, pumpkin pie spice, baking soda, and salt until well combined. Fold in pecans.

Generously coat a 12-cup Bundt pan with baking spray. Spread cake batter evenly in prepared pan. Bake at 350°F until a wooden pick inserted into center of cake comes out clean, about 1 hour. Let cool in Bundt pan on a wire rack for 15 minutes. Invert cake onto wire rack; remove pan, and let cake cool completely, 2 to 3 hours.

Make the frosting: Whisk together cream cheese, powdered sugar, vanilla, and 1 tablespoon water in a medium bowl until smooth. Whisk in remaining 1 tablespoon water, 1 teaspoon at a time, until desired consistency is reached. Spoon frosting over cake. Garnish with pineapple flowers and fronds and edible flowers, if desired.