I want to send solidarity to you OP and sympathy.
I strongly suspect I'm on the less severe end of the ADHD spectrum. I pick my skin constantly, find everything boring, am horrendously disorganised. I don't have a good job and I'm only good at the 'firefighting' bits of it. The boring admin (like as you say mileage claims) I simply can't do in the way I'm expected to - always behind deadline. I excelled academically but lost my interesting job after having children (and due to DH's career - long story) and ever since have been bored to tears (literally, I cry a lot at times).
I agree with what you say. Whilst I may well be ADHD I'm not at the point I can't cope at all - I struggle to cope, yes, and feel like I'm only just keeping my head above water all the time which I guess is 'masking'. I have small to medium things I fail at, but I don't have loads of debt (other than mortgage which I do have) or serious problems from it.
If I told anyone other than on here I think I'm ADHD then I'd probably be in that group you describe of people who appear to be coping. I forget to take my medication sometimes,(not ADHD meds), despite alarms and reminders (and it's important I take it so not good this happens) but I'm just about coping. I'm on HRT which has not helped at all and I do think peri / menopause made it all worse but I've always been like this to a certain extent and there's huge ND in my family. As a girl and compared to the more obvious struggles of the boys, growing up, it was all dismissed and jokes made like 'Ginny's always late' 'Ginny's messy' etc.
I do feel that there are so many people claiming ADHD and ASD that it actually makes it harder for people who can't cope because of the severity of their conditions and for whom there are serious consequences. And also much harder to access support too because there's only so much to go around, and this is the real unfairness.
So on the one hand I sympathise and empathise but on the other I'm clearly not as badly affected as you are OP, I'm on the milder end of the spectrum. I don't think it's the same thing at all and honestly it's really sort of unhelpful that different ends of the spectrum are called the same thing.
Your example of that is a good one - it's like if 'rich', 'middle-income' and 'poor' were all called the same thing and it was just a question of degree of amount of money - a 'spectrum'. But obviously the outcomes and lives of people who are very rich, middling or poor are entirely different. There's a big difference between being able to feed your family or not, or pay your bills or not.