That's fairly extensive testing. Not sure why they want to re-do the cognitive assessment, but I can speculate that they might use an aptitude-achievement discrepancy formula that may require a specific cognitive instrument to be used (for the regression or linking tables). WISC/WJ is not the most common pairing for ap/ach discrepancy. (The publisher provides linking studies for WISC/WIAT and WISC/KTEA.) There are normative tables for WJ/WJ, and linking studies for SB/WJ. So that might be it. Or the neuropsych may not have administered the entire WISC (not unusual for NPs), after obtaining sufficient information, and the school wants a comprehensive cognitive battery (say, for the above ap/ach discrepancy, or PSW (pattern of strengths and weaknesses)).
An extended eval is a mutually-agreed on additional time to assess other areas, based on questions that arose during the initial evaluation process. In the meantime, an IEP is written so that services can begin immediately on signature, without waiting for the additional evaluations to be completed. This is an option when there is agreement on eligibility, but uncertainty about specific services. Some states have set procedures for this.
