The SP is not allowed to make any accommodations, as they would invalidate test scores (tests like this are the documentary basis of many accommodations).
Your DS already has an IEP for motor skills, so it would not be surprising if the lower scores in the PSI were simply reflective of fine motor delays. Those same motor delays would affect the VSI tasks, as both of them are motor-involved, and include bonus points for speed (which, given his PSI, he probably didn't get many of, even if his designs were accurate). Curiously, he did better on the (mildly) motor-involved WMI task (Zoo Locations; involves placing a card, but isn't timed) than he did on the motor-free task (Picture Memory). The former is a visual-spatial memory task, and the latter a sequential memory task, for which either visual or verbal memory (though more visual, for most children) strategies can be used.
This answers your question about his mental processing speed vs the PSI: this is a fine-motor speed task, so a lower score may be due to mental processing speed, fine-motor speed, or something else (there is a long list of options, but since you've already documented fast mental processing speed and motor delays, it makes sense that the issue here is fine-motor speed).
Even the GAI would probably not be the best representation of ability, as it would be affected by the (probably) lowered scores in the VS cluster. And some of the FRI subtests aren't that high, either. I'd be quite interested to know if the evaluator probed for/commented on the rationale behind the responses he gave on Picture Concepts, as I've observed divergent thinkers scoring low on this one in particular, because they identified plausible, but non-standard, conceptual categories. Thereby receiving no credit for those items.
I would agree that some of your numbers are odd (assuming these are US norms, and that he was assessed at age 5). The top score obtainable for a five-year-old on the WIAT-III ERS is 141 (and that's for a very young 5 yo). The max score for a mid-5 yo is 135, and for an old 5 yo, 130. A late 4 yo, OTOH, could max out at 145. That being said, the subtest scaled scores do not preclude the FRI and VSI being accurate numbers, as index scores are not straight averages.
The relatively low ceilings, BTW, are because the most difficult item on ERS requires matching two words to a picture. Any child who can read at all should be able to max out the subtest.
It is not entirely clear how much of his math ability is not documented on the WPPSI, as his FRI and VSI (even with some likely score depression) are both plenty strong. His WIAT MPS score is very high, but also may reflect the extremely low expectations for PK and K level students. To score a 160, a 5 yo needs to be able to do simple arithmetic, and possibly some simple fractions, basic money, time, and calendar. Truly abstract mathematical thinking is so rare that it is not even assessed at this age. The kind of math skills he currently displays still have a strong language component to them (in the sense of the elementary vocabulary of math), which actually lines up fairly well with his exceptionally strong language reasoning.
To sum up: as to whether he has a disability being masked by his strengths, I believe he already has an identified disability, which generally appears to line up with the lower(ed) areas on this assessment. As to inconsistencies with his IRL academic presentation, I don't really see gross ones. And he is, of course, still very young, at an age when testing results can be easily affected by many small-child related factors. Generally, cognitive assessment numbers are not considered to be stable until around age 8 or 9.
