As you say yourself, you know little about cars and the assessor (whom presumably does) has agreed the estimates. On what grounds can you argue the claim is excessive in that scenario? I was rear-ended myself once. I thought it was only a matter of replacing the bumper. In fact, the wings had to be realigned, the panel under the bumper replaced adn other bits and pieces done. Then it had to be undercoated resprayed to match the oriignal paint, wheels balanced, chassis alignment checked, etc..
What I'm saying is that, given my experience, your story does not sound like OTT regarding cost/time.