I'm so sorry for the loss of your dog. I agree that training your heeler first and doing safe tests would be good.
If it makes you feel any better, I don't think gemy was trying to make you feel bad at all...just commenting on how dangerous they can be. I say this because I didn't even know they were a danger on walks before I read this post!
I'll make sure to never use it on walks, and only use the regular leash. Thank you for sharing your story. It's going to help my yorkies be safer now!