I haven't read the whole thread, so apologies.
dd was diagnosed as autistic at age 16. She is very clever and academically has always done well, and likes to please teachers so right up until she was 15/16 she coped. School couldn't see anything wrong even though all the signs were there in terms of social skill and friendships, and rigidity around routine and so on. Then the pressure of GCSE's pulled the mask right off and she crashed badly.
I had suspected she was on the spectrum for a while, but it wasn't until we did her diagnosis and I listed all the things that I really saw how much she was struggling with real life.
She has 2 cousins who are also on the spectrum. In the light of their diagnoses my dh's familiy have all been looking at themselves and their parents and wondering who else.
The thing is they all have loads of traits. My FIL was one of 10 and when I look at the aunties and uncles now I laugh that we didn't see it. All 10 of them are probably autistic to some degree.
But they all have lived successful lives, holding down jobs and marriages, and raising kids. They chose hobbies and lifetsyles that suited them and generally their daily lives have not been massively impacted.
Would they have got a diagnosis today? I don't know.
My dh is now curious about how his traits and personality mix, and if he is actually autistic or just has traits. My SIL was badly burnt out and had a lot of stress related illness when her kids were small, and looking at that now, and at her, it is likely that that was autistic burn out, but she doesn't yet have a diagnosis (although she is persuing one)
My older dd also thinks that they are ND, more likely to be ADHD than autism.
But only younger dd was affected enough that she crashed at school and struggled to do GCSE. Now doing A levels, she needs masses of support.
So if the criteria is that it has to have an adverse effect on daily life, then only dd and her 2 cousins qualify. If we are talking about traits, then the whole family is riddled with them.