Make the bombe: Line an 8-inch metal or glass bowl with plastic wrap, leaving a 4-inch overhang; place in the freezer to chill. Cover the outside of a 6-inch bowl with plastic wrap. Beat the heavy cream in a medium bowl with a mixer on medium-high speed until stiff peaks form, about 2 minutes. Fold in the vanilla ice cream and mashed bananas. Remove the bowl from the freezer, spoon in the banana ice cream mixture and press it up the sides in an even layer. Press the smaller bowl into the mixture to help keep the shape. Freeze until the ice cream is firm, about 1 1/2 hours.
Let the chocolate ice cream soften slightly. Remove the smaller bowl and plastic wrap from the larger bowl. Scoop the softened chocolate ice cream into the cavity; spread to flatten. Freeze until firm, about 45 minutes.
Let the strawberry ice cream soften slightly, then scoop on top of the chocolate ice cream; spread to flatten. Make a large indentation in the center of the strawberry ice cream with the back of a large spoon; fill with the cherries and press to flatten. Freeze until firm, 1 to 2 hours.
Remove the bombe from the freezer; invert onto a plate and remove the bowl and plastic wrap. (If it sticks, run a small spatula around the edge to loosen.) Return to the freezer.
Make the toppings: Bring 1 inch of water to a simmer in a medium saucepan over medium heat. Put the chocolate and coconut oil in a medium heatproof bowl; place over the simmering water and stir until the chocolate is melted. Set the bowl aside until the chocolate is slightly thickened, 5 to 10 minutes. Slowly pour the chocolate over the bombe, allowing it to drip down the sides. Top with the walnuts. Freeze until the chocolate hardens, 15 minutes. Let stand at room temperature 10 minutes before slicing.