Misty,
Certainly, you can add some butter to your buttercream. It needn't be ALL shortening. If you do 75% butter, 25% shortening, that's plenty of shortening to stabilize the buttercream.
That said, you can use flavored, tinted buttercream as an icing, without fondant, OR you can do the same with the fondant.
Instead of liqueur, use orange juice concentrate for some extra dimension and moisture.
I really do think that a citrus curd can give you the flavor and heat resistance that you're looking for as a filling.