Oh I am so glad Blossom is alright and how scary for you. I don't like collars for this reason and others but I use this harness on Ftse and had no trouble with it coming off< I used a easy walk harness on him and it fell of him while walking along the road so that went straight back to the shop and I put him back into this one it is an EzyDog.
This is the identical harness I use with Ace. Adjusted properly I have had no problem with him being able to back up out of it. It might be possible but would be very difficult.