Were all on pins and needles, what happened to the dog? I hope he's okay.
And btw, my car front is DAMAGED to the dog so if I were to go to the owner then I would freak the hell out for 1.) not giving two craps about letting their dogs run free after fines and 2.) I would be making the owners pay for my damaged car.
Okay, I understand your upset about your car being damaged and the accident, but try to remember this is an animal that does not know the difference between right and wrong.
So, you haven't talked to the owners about the accident? Shouldn't they know if their dog needs medical attention or is dead. I absolutely understand your frustration with the owners. We have the same situation at my house with dogs on the loose, but if I hit one I would still let them know so they can handle the situation.
To answer your question, I don't know about the seat belt thing. I'd call your doctor so they can set up an u/s if your concerned about it.