DS10 is asking very reasonable, thoughtful questions. It is, of course, impossible to absolutely answer his question about how he would have performed differently if he had been aware it was an IQ test, but I should point out that most examiners do not introduce the WISC to children as an IQ test. ("Learning style," "learning ability," and "helps us understand how you learn" are all phrases frequently used.) Also, (and part of the reason why this is the case) it is quite possible that he would have posted poorer performance with that awareness, depending on what role performance anxiety plays in his life.
To the question of examiner skill, naturally it's difficult to speculate on that too. So unfortunately, his specific questions don't have firm answers.
But there are some general comments I can make about 2e GT/ASD kiddos: many of them become more conventionally testable as they mature, with performance, in my experience, rising sometimes all the way through the end of high school (despite regression to the mean). Perhaps one way to discuss this with your DS is by reflecting that norms are based on the general population--which is, by definition, neurotypical. So if one happens to be twice neuroatypical, it should not be too surprising if standardized scores have a harder time fitting one's profile of strengths. (This is a bit like how using norms developed on English monolinguals to assess English learners introduces confounds that tend to underestimate their abilities even in supposedly nonlanguage domains.) ASD learners may become more conventionally testable as they mature because they learn more of the expectations of neurotypicals.
And on little DS's upcoming testing: that is a totally valid caution regarding normative performance. Relevant to both DSs' performances is the reminder that, after all, test results are only one sample in time of a subset of an individual's skills, not some sort of fixed absolute number. For the moment, it's not as important whether little DS's scores are representative of his optimal or "true" abilities, as that they reflect (more-or-less) his current levels of function. The purpose of the evaluation is to assess with respect to autism. Assessment of intelligence is secondary at this moment, and typically more to rule out cognitive impairments.
