When using the toggle "Shape Key Edit Mode" it gives me a representation of all shapekeys mixed together but it's only a visual preview.
It doesn't affect the result of rotating or scaling the mesh since you still only work on the shapekey that is selected instead of the mixed representation that is shown.
And since it's all just based on vertex location the results can be pretty odd, especially when using the 3D cursor as a pivot point.
Since working with multiple shapekeys is a great way to work non-destructively it would be a big improvement if the mixed shapekey representation would also be used for any deformations made.
This could be by default or as an additional toggle.