banana pudding mousse

You need to keep a cake filled with milk-based banana pudding in the fridge at all times.

If this is for a customer, your state food laws (California for one) may prohibit its sale altogether.

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.

Well you don't have to throw anything away.  What size was the banana pudding mix? You can add part or all of that second package you already have, pudding rescue works well even if it's the kind that you cook. Use a mixer to get any lumps out.

Buy whatever you need for the second filling--the proportions are 1 package instant pudding (making 4 servings from 2 cups milk) and 1+1/2 cups whipping cream. For your peace of mind, I would buy 2 packs pudding mix and 3-8 ounce cartons whipping cream.  If the store has it, pick up some rum flavouring and try a little bit with a spoonful of filling.  You can also add that rum flavouring as part of the cream in the whipped mousse filling.

Mixing in buttercream:  I'm still not sure.  I would make a stiff whipped pudding and just use the buttercream for a good dam around all the fillings like this http://www.wilton.com/cakes/making-cakes/filling-cake-layers.cfm

Now about sliced bananas...you can use them if you put thin layers of pudding on both sides of thin slices.  You want the banana slices to be completely enclosed without any air spaces betwen them and the cake. No need to dip into lemon juice or anything else...just slice 1/8" thick, and seal them in completely.  Chill the cake for a good hour after you assemble layers and crumb coat,  before you add dowels--then they will go right through the slices and your cake will not collapse. 

Once you stack up all the cakes for each tier, use some plastic wrap folded in half lengthwise to wrap around the sides of the separate tiers.  Wrap so that the cakes are firmly held together with the fillings, and chill them for 6 or more hours. The pudding will slowly begin to hold the cake together even if the layers are crumbly now.

If possible, ice the tiers just before you go to bed and let them chill overnight.  This will help to stick the cake layers together  for decorating.  Take them to the party as two separate tiers and stack onsite.