You can't remove the yellowing. It's caused by bleach. Get a new seat and don't use bleach on it and it will stay white. Clean with tea tree, lemon, and/or white vinegar. Limescale is a nightmare. You can only really keep it at bay. I used distilled water in my iron and a kettle catcher (like a wire ball) in the kettle. I'd use white vinegar (and 10 drops of essential oils) in the washing machine to help keep it at bay - this acts as a softener and will break down the limescale. I'd do a limescale treatment on everything first before you implement new things so that you're starting with the least amount of build-up possible