I am new to this site and to the issue of giftedness and the WJ test. I have many of the same questions that you have, and I found the following paper/article to be very helpful:

http://media.wiley.com/product_data/excerpt/90/04714199/0471419990.pdf

The paper does a very good job of explaining the terms used in a WJ report, and even includes a few examples of sample questions used in each section.

I think you'll find the article helpful. Dan