Cake in fridge--OK. So you are looking for a different stripe in the middle. You didn't say which buttercream, so I cannot say anything about your idea of mixing it with the pudding--it might fall apart.
If you want a fluffy white mousse, use 1 package vanilla or coconut or white-chocolate instant pudding mix and 1+1/2 cups whipping cream. Beat that up and spread it promptly, it will hold the cake together well.
You might have to use part of your second package of banana pudding for your other two layers of filling. You didn't say how much milk you added. For a cake filling, you need to make the pudding stiffer than the package says by adding 1+1/2 cups instead of 2 cups milk. Spread pudding filling 1/4" deep.
If you want to stick with banana flavour, you can make a banana "jam" by cooking mashed bananas with some brown sugar and rum. 1 cup mashed banana, 3/4 cup sugar, 1/4 cup rum and simmer until it gets thick.
And you will need to use large straws or stiff dowels in the bottom tier for sure. Use a stiff cake circle (corrugated cardboard) under the top tier.