Answer to Question 1
Both must be combined to apply them globally. Both models assume a very weak crust and faults that penetrate completely through the crust, to the lower crust. This assumption is contrary to the reality that the upper crust is strong and the lower crust deforms plastically. Both models assume the solid rock mantle behaves like the water used in the model. Although hot mantle does flow like a liquid, it is slow motion due to the high viscosity of the liquid.
Answer to Question 2
If you change the mass of the isostatically balanced column, then there is a difference in mass between that column and the adjacent columns that must be balanced by elevation. Erosion and deposition can decrease or increase the mass, again requiring adjustment.
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