Families generally see a case manager every few weeks as they tend to be expensive to hire so you might not be able to sit in a workshop with the case manager, but during the initial visit which normally lasts a few hours, you can ask them anything and everything, they will also meet the child and try to engage them etc. we had three consultants visit us at home when we were starting out and after talking to them we knew instantly which one we wanted to go for.
The case manager should give you details of the families they are currently working with and families are normally very willing to help and most of them would be happy for you to come and see the actual sessions with the tutors. I would definitely recommend talking to other families and taking up any references the case manager/tutors are offering.
Waiting is the hard part, I understand exactly what you mean by seeing your child slip away. The good thing about that is you can do something about it. Seriously while you are waiting to start ABA, buy the book "More than words", apply its teachings and you will definitely see a difference.
Don't worry about imaginative play at the moment. Go back to basics, peekaboo, rough and tumble play etc. worked best in engaging our ds. Keep the play simple and play alongside her. Encourage her to join in, but do not force her.
cause and effect toys are good, something that lights up and sounds when pressing buttons are good. See what she likes and join in.. keep the language simple and even though she might not seem interested now, it will pay off soon.
I don't know much about your dd's issues whether she is verbal, anxieties etc, so would hesitate to recommend other interventions. A good ABA program should incorporate elements of OT, SALT etc but if you can see a SALT privately who specialises in ASD, then that would be useful. HTH