If you want to obtain the original walls back before the split operation, you can use the option “undo last wall split”. This can be done all at once, in small groups or one by one. If you encounter any difficulties when reversing the splitting operation, try doing it in small groups.
If it doesn’t let you add more wall subtypes, it’s because it has already reached the maximum number of layers.
If you want to select the exterior walls using the “smart wall selection”, make sure you are in a floor plan and not in a 3D view.