Sorry to hear about this - I've not got direct experience of the process but friends and family have been through it with their kids and it's hard. It often takes a crisis or luck or good connections to get the help needed. And it's not just this - I've helped my uncle through health and homelessness, I've helped my Dad with disability related applications and my Dad was previously part of a local advocacy charity that helped people with health and benefit issues. The story across all that was pretty consistent - everyone is overworked and there isn't enough money, so they will look for anything to knock you back.
In some ways you are being penalised for having your life together (although I imagine it has taken a tonne of extra work and an understanding partner just to keep the wheels on). If things had been in disarray, that would have counted as a big sign something is wrong and requires intervention.
Also:
I had my screening interview back in June, and that was followed up by two web-based questionnaires, one for me, and one for an adult who was about when I was young. My mum filled this role.
This kind of thing needs a lot of focus and effort to get right. If you both don't hit all the right points and if you can avoid the boobytraps, you will have a stronger case. I'm not suggesting you lie but you need to ensure you present the beat possible case and we British tend to not to want to.make too much of a fuss when, in cases like this, we really need to be rattling the cage bars and shouting. I've been over replies to such forms with a fine tooth-comb and you really need to make sure it is very focused on getting your diagnosis.
A guy I’ve spoken to a lot who has some experience says my lived experience sounds fairly textbook ADHD/autism as taking effect from my mid to late teens
I wonder if you should try reapplying but for autism initially and, once you are in, you can expand the focus to other comorbidities. It seems like the diagnostic criteria are clearer and it is less likely you'd lose points for being high functioning as it is a known thing.