Yup..I agree with the plain and SR flour thing.
Although they are basically the same thing - I tend to follow what the recipe says and then it always comes out. Now to contradict myself - with the muffins - it really should not make any difference at all...biscuits might.
The cornflour is a thickening agent and plain flour will not substitute. The rice flour I am not sure...if it were me I would use pl flour instead but def use the cornflour.

With the sugars - castor and icing have very different textures which certainly do reflect in the finished product.