In addition to everything mentioned above, I would first say sleep/sleep hygiene needs to be fixed and not just accepted as "chronic baseline." Everyone gets irritable from sleep issues, especially teens with developing brains. Even more so, autistic nonverbal teens.
I'd put the onus back on the parents and insist on family therapy (without the child). It can help with many different aspects: Parents need to be held accountable for their child's sleep schedule as above. They need to learn coping strategies for dealing with an autistic teen. Med are not a substitute for parenting, and an autistic child requires extensive, skilled parenting. Parents also need to grieve and process the fact that their child will never be "normal" and set whatever hopes and dreams aside to move forward and deal with their child as they are in the here and now. Basically, acceptance of their child who has a neurodevelopmental disorder (and that some degree of random outbursts/aggression is their child's normal).
The child's low function and being nonverbal -- have you considered intellectual disability as the actual diagnosis? Or comorbid with autism? Parents tend to get overly attached to an autism diagnosis when they can't accept their child is ID.