Well balanced and well written. So glad you brought up tethering.
When one of my twin daughters was younger, she had seizures and couldn't walk for a while and then when she could she had balance issues due to CP. I know having autism is a different set of challenges. But looking back. I realized we could probably had a service dog for my daughter's issues, but having an extra something to take care of and then worrying about transporting the dog, and walking the dog just for its own health an exercise, the stress of that responsibility would have put me over the edge. There is much to consider.
On the other hand, once she walked better and the seizures stopped, she was able to fully enjoy a dog as a pet. Having a dog to walk and care for gave my daughter a great deal of independence. Having a dog was the best therapy ever and I wouldn't discourage any family with an autistic child to not have a dog, it could be very beneficial it just does not have to be a service dog.
I'm sorry to hear that adults with autism are having difficulty in finding service dogs.