I just wonder how others see this and whether they think the same way as me.
Hiya Mati
My own view is that if a person is responsible for an animal, and something happens to that animal, then the responsibility remains with them, unless the car driver drove into the wood, or onto the pavement (sidewalk

)
In this case, your nH took the lead off the dog too soon. End of story.
It is his responsibility for the dog turning and running into the road. The dog shouldn't have run into the road, but cannot be blamed because that is the kind of things dogs do; that is why we put them on leads.
The driver may have been going too fast; we don't know. Your H says it was 70mph, but he would say that even if it were only 30mph. I think that had it been a 70mph impact, the dog would have been killed. Some of our road adverts here say that if a child is hit at 30mph it has an 80% chance of surviving, whereas at 40mph only a 20% chance of surviving. So perhaps we can realistically say the car was probably
not doing 70.
The car driver was not a hit and run. He stopped, and he was perfectly right in saying that the dog should not have been loose on the road. But your nH would not accept that, and switched the blame onto the car driver, and showed anger and animosity. At that point, if I had been the driver I would have done what this person did; got back into my car and driven off. I would have been sorry to leave the dog behind, but too afraid of the owner to stick around. I think that is understandable, whether the driver was N or not. (But I suspect an N driver would not have bothered to stop the car.)
I am very pleased the dog is ok, but I don't think the Ns involved will ever admit the truth. They don't know what truth is.