Good questions. Some semi-organized thoughts below.
The intent of disability-based accommodations is to so-called level the playing field. No student has a right to A+ grades, but every student does have a right to equitable access to their education. Federal regulatory advisement currently takes the position that this access means something better than minimal, but not quite as high as optimal. A private institution, of course, (one which takes no federal moneys) can make its own decisions.
Another, perhaps more pragmatic, way of thinking about some of the accommodations you mention is, will he have these accommodations in the future for high-stakes assessments. E.g., will he have these on the SATs? The major testing companies make their own rules for accommodations...but also increasingly have assistive technology embedded in computer-administered tests. For example, the SAT is now a semi-adaptive test (by item set), which should shorten the testing time for nearly everyone. 4-function calculator is now standard and in the platform. AP exams have a testing site option where the school actually can choose to give typed response to every student. So it might be worthwhile to look ahead a little (keeping in mind these are all in flux, and could be quite different in another 3-5 years when your DC is likely to be testing) and identify which accommodations are most likely to be relevant for him, and then focus on those for advocacy now.
In addition to your eval report, accommodations also should take into account their effects in the classroom, and consider which ones are really the relevant ones. For example, if the checking of work on a math task is because of messy calculations, then is it possible that he would benefit more from completing his work on square/gridded paper as an accommodation for reducing the impact of messy/poorly spaced/aligned numerals, thus preventing the careless errors? This is not to say extended time isn't appropriate, but simply to give him additional tools for managing different scenarios, and to focus on the underlying causes.
Along related but slightly different lines, consider whether possible perfectionism is more of a functional concern than careless errors are. It sounds like he is placing a lot of weight on single measures as determining his access to desired/necessary resources (in this case, advanced math). Does he truly believe that A- grades are merely adequate? Or that getting a B in the class will keep him out of higher-level classes? If this messaging is coming from adults (such as imposed by administration), it may be worth having a conversation not only with your DC, but with the adults around him regarding the relative health and likely long-term outcomes of a child with Bs and a flexible, resilient, growth mindset, vs a child with As and a rigid, anxiety-ridden, perfectionistic mindset.
