Who bears responsibility for damages if a car backing out is hit and back bumper is knocked off?

Generally, the car that is backing out bears the responsibility of granting the right of way to cars driving behind it passing them.  if the car that was backing out had its bumper completely knocked off, then it is fair to assume that the person backing out did not see the car coming by and "T" boned the other car.  however, i would have to see from your drawing as to where in fact the car not back out was at the time of impact.  Was there a police report?  if so, notwithstanding my or anyone else's opinion, the police report wil control as far as the insurance company is concerned and will be very influential at a trial.  I sugging hiring a lawyer to review this is detail.  most lawyers will meet with you for free the first time.