What do the simultaneous up and down arrows mean in the Summary Tables?

This simply refers to the fact that the mutation's effect must be considered in the appropriate context or on a case by case basis. For example, the GST and SULT mutations affect the substrate binding site changing how it can fit with other chemicals. That fit may be more difficult or easier depending on the shape/conformation of the molecule it is trying to bind with for enzymatic processing. Therefore some compounds will get processed faster, while others will be slower.