Well, it sounds like she is exhibiting a very strong burying instinct.
The problem appears to be a mis-fire in the signals telling her to scratch/bury ... because she is scratch/burying the wrong thing ... but she IS scratch/burying, so her instinct is perfectly fine. Unfortunately, we can sometimes modify instinctual behaviors but we can never completely eliminate or change them. You may be stuck with her wall scratching and not effectively covering her waste.
I've heard of cats scratching air, walls, floors and even walking down the hall away from the litterbox area and stopping every few steps to scratch/bury at the walls in the hallway ... when their l'box/waste are 10' behind them! Instincts are funny things.
In our multi-cat home we built a LitterChest that is huge and vented outside so most dust and odor go out of our home, whether the waste is buried or not. Sometimes other cats coming into the L'Chest will bury an uncovered poop if it is still fresh/stinky, but if it has begun drying, they just avoid it. My kitty who doesn't cover her waste either scratches at the walls or does absolutely nothing when she has finished other than to rocket at top-speed out of the L'Chest, down the hall and into the livingroom to mock-attack whatever cat she sees into playing with her.
Kitties seem to get strange ideas into their heads, but those ideas never seem strange to them. Just us.
Best of luck!