To reset walls to their default materials using the material painter choose the option Use Default Material at the bottom of the Select Library Object dialog.
There is not a defaults dialog for stairs. Instead, the default materials for stairs are specified in the General Material Defaults dialog.
Reasons why materials would flicker include plan placement, large terrain, materials occupying the same space and video card.
In Chief Architect, images can be added to the Library Browser for use in any plan using the Copy and Paste Special option.
Chief Architect's Material Region tools can be used to place different materials on a floor or wall within the same room, such as a walk-in shower.
The Material Painter modes help you specify exactly what is painted when you use the Material Painter tool: Component, Object, Room, Floor, Plan.