Don't sign any release unless you are COMPLETELY satisfied with the outcome.

In order to push a middle car into you, that rear guy must have been going way too fast for conditions.

Talk to your insurance, explain the situation and make them help you chase. It is possible your UM/UIM coverage may be called into play if the Enterprise guy didn't get the insurance, so better to get them notified now.

Also your policy's med pay will come into play immediately, so if you suspect any injuries be sure people know that now.