Staying on topic, and I'm glad to hear that your wife never suffered lingering injuries, however, I'd like to argue the point from the drivers lawers view.
You say you heard a bang, and saw your wife remonstrating with the leaving car/driver?
So, the car was reversing, and how fast was it going in reverse, for you to hear an audible bang?
Your wife was still stood up at the time of, and immediately after alleged impact, and (she) had hit the back of the car to stop/warn the driver?
I put it to you, that the 'bang' you heard was infact the result of your wife hitting the car to stop/warn the driver. Further more, injuries sustained - pulled muscles/bad back strain - could have, and most probably were caused by the effort of your wife quickly turning towards and striking the rear of the car with enough force for you to have heard the 'bang'.
This hypothasis can be proved by the pain being regional to the (alleged) impact area. i.e, your elderly mother was on the right* (assuming) as she was not in collision with the car also. This indicates that the car was likely to strike your wife on the left* side. Rapid turning to the left*, to strike the rear of the car (human natures indicates that you turn towards the danger and not away from it) with her right* hand, would cause muscle damage on the right* and bruising occuring with the muscle damage.
Though my client accepts that he was careless in his reversing manouver, he did not infact come into contact with your wife, and that the injuries caused were due to the actions of your wife.
* left/right directions are dependant (in this scenario) on the postion of your elderly mother, whom you state was being aided by your wife.)
I think, should any insurance company read carefully the statements given by any legal representative of the driver, then you are likely to receive a minimal award.
As for leaving the scene of the (alleged) accident. Police will not even bother to attend the scene of an accident if there are no injuries caused during the collision. As no physical injuries were reported at the time, and your wife was not reported to have been 'down'. Then it too can be argued that the driver left, after receiving the hit to his car, and left to prevent possible further damage caused (if any) to his vehicle, by an angry pedestrian.
Reading a story from an angry author always leads to distortion of facts.
Again, sorry to hear about it, but it happens every day in most towns/cities.
p.s You might find that should your wife wish to pursue a claim for injuries, the driver could (and it does happen) puts in a counter claim for damage (however minor) to the rear body work of his vehicle. If it can be shown that there is a dent where your wife struck the rear of the car in her anger.