I think you need to consider the possibility that he doesn't appear to have problems at home because you are exceptionally good at managing and preventing issues. Grinity, who doesn't post much anymore, mentioned more than once that you should never underestimate the ability of a gifted parent (mother) to compensate for their 2e child and make them look "normal" at least some of the time.
I know too many kids wih ASD or ADHD who either behave impecibly in the world and then fall apart at home, or are ok at home but not at school to have faith in the "multiple environments" thing. Sure if he's fine with all teachers except one then its probably the teacher.
Also it's common for kids with ADHD to have either (or both) WMI and PSI low, but neither are a requirement. My DD has ADHD and has WMI in the HG range! And yes she absolutely has ADHD and you can see that the WMI is high in every day life. Possibly her processing speed is low but it's never been tested accurately.
Also ADHD is far more heritable than asthma or breast cancer. If the tester WAS biased its not without reason there.
By all means get another opinion, but please consider the possibility this is real.
I'll second all of what MumofThree has said. I do feel that it's very important to listen to your gut feelings, but at the same time (as the mom of 2 2e kiddos - but none with ADHD, so there's my disclaimer) - it's also easy to *not* see the forest for the trees.
The one thing I'd want a little more info on from what you've posted is the relatively low score on processing speed - I think you mentioned fine motor issues? That would impact the processing speed scores (the ps test rely on responses that are drawn and they are timed tests). Fine motor issues *might* be causing frustration at school which might lead to behavior challenges with school or homework and possibly tasks outside of school.
But if it was me, as a parent, and my dh had an ADHD diagnosis that I didn't question, I wouldn't be too quick to toss out a diagnosis of ADHD with the symptoms you've mentioned.
Best wishes,
polarbear
ps - I also forgot to add - did the neuropsych eval include any testing to tease out why the issues with processing speed? While it could be fine motor, it might also be a student simply taking his time, or it could be an issue with vision or visual processing. There are tests that neuropsychs typically run to determine why processing speed is relatively low when it deviates as much as your ds' scores do. One of my kiddos (not 2e), scored in the basement on processing speed relative to her other scores on her first WISC. She is also a highly sensory kid who *looks* very much like she has ADHD because she likes to be in constant motion. The low processing speed score combined with the additional neuropsych testing to determine why processing speed was low pointed clearly to a vision issue - and sure enough, she wasn't able to focus on anything! She went through vision therapy for more than a year, and most of her behaviors that looked like ADHD disappeared. I am *not* suggesting that your ds has vision issues, but just suggesting that there is most likely more useful information in the root cause of the dip in processing speed on his WISC. If he had any additional testing with the neuropsych, or if you can post his subtest scores here, we might be able to help with some additional thoughts.