It happens a lot where I live, due to many hills.
At the bottom of my road, an Audi TT rolled backwards down a hill across someone's front garden and smashed a window of the house.
Me being a bit slow sometimes, wondered why they were having DG fitted on a Sunday. It was replacement window for the one smashed by Audi TT.
Leave it in gear
Also past there at a blind junction that you have to inch out from gingerly and directly opposite in a driveway there is now a 159 Estate parked there nose towards taunting me to join the Alfa sect.
Regarding the garage, try and be calm and measured and say "I've spent a lot of money here on getting things fixed, that remained unfixed".
You could put the ball in their court and say "how do you propose we resolve the situation?"
You could over egg the tale of car rolling away...."the car could have been damaged, someone could have been hurt"
Years ago I had a brake failure after having a major brake refit, it was in the days you could get legal aid. They settled the day before it went to court.
"It'll be all around work that the blinkered Alfisti almost had their lemon totalled as the handbrake didn't work - it's embarrassing for me"
I think you need to take the "I've spent a lot of money and it still aint right" tack, as it's too easy for them to say "leave it in gear".
If it's any consolation I had something similar happen at work in front of a few colleagues, I left van in neutral and hand brake off and it started rolling forward across yard. One of those colleagues that witnessed this is rather splendid, but I really frown upon fishing off the company pier, as it can make things complicated.