Clearly, her verbal achievement (per multiple WIAT reading subscores) is at the gifted level, and you haven't been "hothousing" the WIAT skills, so in your situation, I would feel very comfortable continuing her in extension activities and taking the WIAT scores as circumstantial evidence of high ability, since you have good reason to believe that the WISC scores were depressed by previously diagnosed anxiety and selective mutism.
It's not fair to her to use a test to measure her intellectual capacity where the format of the test itself necessarily measures her disability at least as much as her ability.
