Why not apply for a completely trained hearing dog? I just turned in Jack (1L8) BL/M to Southeastern Guide (all post a picture link later in my intro) they specialize in guide dogs for the 3.7 million Americans who qualify for a guide dog due to VI/Blindness but their
Gifted Canines Program includes:
Hearing, Mobility, Seizure (not the proper term I think), Search and Rescue, Cadaver, Arson, Bomb Detection, Drug Detection, Anxiety, Therapy*,and other such dogs. (Therapy dogs are NOT granted Public Access under the ADA)
The dogs are always insured by Southeastern Guide (or whatever Service Dog school) until retirement so IF the dog ever did anything YOU would not be liable as long as you are following their rules (which for graduates are not bad at all)
You are in SE area of placement I think. The difference between how your collie currently helps you and a hearing dog is that they are trained specific behaviors for specific sound events.
Sirens behind you when driving, a smoke alarm, car horns - all have different specific responses.
I they have a website
Southeastern Guide Dogs - Home but it omits information about the Gifted Canine Program... so I don't know how to get you more information on that.