Seperately to this it does seem like there would be a great business opportunity to set up a specialist forum for those with disabilities and their families - it could have all sorts of expert guest blogs, practical help and support, get sponsorship from charities etc etc
You're right in one sense but in another sense you're absolutely wrong. Disability should be mainstream. All the things you mentioned should be available from 'able bodied' sites.
Mumsnet have a responsibility to create an 'accessible site' (in the sense that it is inclusive and ensures that people with disabilities are not discriminated against, abused or excluded) in the same way that every other public body or business does. Disabled users are completely within their rights to demand that.
You wouldn't think Tesco could shrug their shoulders and say 'start your own supermarket' if, for some reason, their products or premises were found to be offensive or inaccessible.
If you make money out of the public (doesn't matter if the forum is free) you take on various responsibilities that the UK has signed up to in relation to disability, GLTB issues, racism etc. (although less so post-brexit).
What you and others like you should remember witch and waitrose is that it's very hard to be disabled and you simply cannot understand how bloody-minded and confrontational it's necessary to be unless you've lived it. Rights for disabled people may be enshrined in law but the reality is that most businesses and public bodies dodge it until they are publicly challenged about it in front of the able-bodied world, or the Equality Commission. Disillusionment is the first experience of being disabled; since you represent costs, hassle and potential legal issues, disabled people aren't valued customers. They aren't shown the 'nice' side of businesses or establishments. Most organisations aren't as caring and accommodating as their image would suggest and mumsnet is no different, it would seem.
People with disabilities often have to make ten different phone calls and quite literally read the human rights act in order to have an experience that anyone else takes for granted. It is dehumanising and exhausting to fight like that and you're expected to be grateful to an able-bodied society that thinks, deep down, that they're being awfully nice to lift a finger.
The upshot of all this is that people with disabilities may sometimes appear unnecessarily adversarial. You have no idea of the backstory. As someone who was once disabled and am disabled no longer, I've learnt that you can't judge because you just don't know.