Hi, I know it's very late to the discussion, but I made an account just to say how reasonable you're being! My parents and my in-laws are just like yours, basically insisting that every grandchild with traits they don't like has a disorder or needs immediate intervention.
My son--going into third grade and actually had a brain malformation as an infant. All grands were in denial, insisted that he was fine and would grow out of his delays, weakness, and health concerns. Intensive PT and OT, plus baby neuroplasticity, and he's in thriving in school, learning the trumpet, and playing soccer.
However, he's quirky. He has an extremely limited diet but is adding a food each month or so, gets overstimulated at Disney or live performances, has a temper, and after lockdown, had to see a counselor for a while due to social anxiety. Now my parents insist that he's autistic because he doesn't enjoy time with them, hates talking on the phone, and dislikes being hugged, kissed, or photographed very often.
My parents have diagnosed my nephew with Oppositional Defiance Disorder and a few other things because he insists on wearing feminine clothing, sometimes in public, which they know they shouldn't admit to disliking.
My in laws- They think my son is perfect, but are convinced that his cousin is autistic because she's afraid of hand dryers and doesn't like to run around constantly and get dirty. Also, she is fascinated by insects and will watch them for a long time and try to help them, instead of playing on the monkey bars and "getting exercise."
Two other cousins have been written off as being autistic by the in-laws because they never bonded with my SIL (step-mom) after a contentious divorce when they were very young.
She was polite and highly disciplined with them, including them in family hikes, biking, travel, etc., but never attempted to engage in their interests. They liked video games and TV, weren't academically gifted, wrestled and ran around until told to stop, etc., which both parents allowed in the past and their dad still allowed when he met SIL. She also forbade all junk food and desserts, making no effort to bridge between the two households. With their mom and her boyfriends, they are only junk and had no boundaries, and with stepmom and Dad they weren't even allowed to tell me about their favorite show when asked, because there was no gaming or TV talk allowed.
They did do extensive family counseling, plus individual counseling for the boys, both of whom were diagnosed with depression, but the parents needed counseling even more. For example, years in, SIL still laughed over accidentally making a bike trip so arduous, the boys were crying with exhaustion halfway through. She could've called for a relative to get them, but no. They'd done the ride in exchange for ice cream and they needed to push through just like she did in triathlons.
Zero empathy from either parent, pretty much ever, when talking about the boys, and I'm thinking that behind the veil of kind tones of voice and polite words, that iciness never abated. If anyone is autistic or narcissistic, it's sadly likely to be the three parents in their lives, not the children.
Now the boys are both out after turning sixteen, one kicked out for dangerous marijuana smoking (smoldering ash in trash cans isn't funny, and I get that) and the other run away, but the family blames it on neurodivergence instead of bad family dynamics all around. Nobody has tried to find the runaway or establish contact with either boy, even after three years including a pandemic. They're completely different with parenting their daughters together of course.
Anyway, this novel shoes how comfortable and easy it is, especially for grandparents, to diagnose children and escape all responsibility for appreciating their personalities and interests. It's probably a big thing now because simply demanding that children should be seen and not heard is no longer done.