Why did they take her home then after picking her up from the city centre?
That's what coppers generally do with drunken teens. They don't leave pissed up vulnerable young people to make their own way home. She'll have been checked out over the radio, they'll have found she had no previous and decided to take her home, issuing a postal caution for being D&D or a letter asking her to attend the cop shop at a certain time for a formal caution.
Of course they will have wanted to enter the house. They need to see her living conditions and check that a responsible adult is present. Safeguarding etc.
What happened then? Did she really just suddenly meltdown? Or is there a portion of this incident that wasn't recorded or wasn't released? There will have been two or three coppers bringing her home - one to drive and another one or two to keep an eye on her. Why were extra officers summoned? There must be a reason - they wouldn't use (wo)manpower that is not needed, they don't have the funds. And no, when someone is resisting arrest, they do not care about autism or any other condition. They will do what it takes to detain an offender. Mitigating circumstances are for the court to consider.
I am usually the first to open my gob when it comes to police brutality. I am certain that there is more to this than meets the eye though. We certainly haven't seen the full picture in this video.