What politics? You are bringing up politics…

Pretty hard to talk UK immigration since 2016 and not be brining up politics. The wholer subject is about politics. Hence why.

Also, I never heard of immigration regime being subject to such things as disability anti discrimination laws.

Any law or process passed by parliament is required not to contradict other laws. That is the whole job of the House of Lords. To not be covered by discrimination laws, the immigration laws would need to specifically exclude it. They do not. Nor would the ECHR Treaties allow them to without leaving thos treaties.

Does autism is even count as disability? Maybe this is a UK or EU specific thing but it still does not pass a basic bullshit test.

Yes, it very much is counted as a disability. Historically, more so than now, As in the past autism and Asperger were considered different conditions. We now know they are just different levels of the same condition. This unfortunately has led to many uninformed people seeing all autism under the ideas that it is not a serious mental health condition. When for many it can leave you totally unable to interact with the world around you. Like all disabilities, the level can be very different. But they are still classed as a disability. And just like anyone else, government departments are required to make reasonable accommodations. And the immigration law has sp[specific exceptions to the dependency descriptions for exactly this reason.

The state generally reserves right to deny anyone entry or residency for any reason subject to some limitations.

Utter rubbish. The Geneva convention is just one example of a huge treaty covering a significant % of the planet that denies that daft claim. As it has rules about accepting asylum seekers right down to making any entry method legal is asylum is claimed.

As for this case, the ECHR has specific restrictions of the right to live as family. This means when one member of a family is accepted, dependents and those depended on must also be allowed. It also has laws that have to consider ability when classing a dependent.

Generally speaking adult children are not auto admitted, people with disabilities are not permitted to enter or stay just because they got a disability.

Legally, the level and effect of that disability must be considered under the compelling compassionate exception rule of the immigration act. But your definition ( just because they got a disability). Is sorta correct. It requires the effect of that disability on the dependents' ability to live alone to be taken into account. But the article indicates clearly that this girl has one of the more extreme cases of autism. As many do.

The law can include the exception but presumably discretion rests with the reviewing paper pusher.

No, in pretty much every case within civil service jobs. The paper pusher is required to follow a procedure. And that is why this turned to politics. Just after announcing the election, the Tory party changed those procedures to be more strict. Without changing the laws to match that change. This is not something legal for a government to do. This has been the reason the government loses 60% or more of court cases where they reject disability claims for benefit. (since 2010). Adding the same changes to HMRC is another mess that is just starting to show.

Is there evidence that this paper pusher acted in bad faith or mis applied the law?

The evidence is still to be addressed, but the accusation is the government miss applied the law when issuing changes to HMRC policy during the election period.

Why should “compassionate exemptions” apply to a 19 year old? did they supply evidence that she is in fact dependent on them beyond “we pay for her shit because we got money”

Covered very completely above. The law requires it if her condition means she is unable to live independently, as the article claims.