Value Variance for Totaled 1979 Mercedes

Hello - My name is Scott and I'm trying to help my Mother with an insurance claim. She has a pristine 1979 Mercedes-Benz 450 SLC, which was in an accident. It was not her fault, and she has witnesses that support her. Although she is 80+ years old, thankfully she was not injured, other than a bruised knee. However the car suffered substantial front-end damage and was called a total loss by her insurance. She bought the car new, it has 86,000 miles, and is in excellent condition. Her insurance company originally offered $4,500 and when she protested they offered $7,879.67. So she asked me if I would check to see what the real value of the car is. It's tricky since there are so few of these on the used car market. The NADA price is between $13,000 average and $17,900 high. The Gold Book of Classic Cars says $12,000 average and $16,000 excellent. The few used cars I found online were mostly in fair shape and selling for $6-7K. A couple in excellent condition were asking $12-19K but had only 30-50K miles. The insurance company says they do not use pricing guides, but pay according to the information they receive from CCC Information Services. They took their price, looked around for any used models they could find, and adjusted for mileage. The price they offered is firm and our only alternative is to exercise the Independent Adjuster clause in the insurance contract. I don't think of the car as collectable, but would like to get $10K or $11K which is only slightly more than the almost 8K offered. I would appreciate any guidance or suggestions on how to proceed. Of if this is most likely a fair offer. Thanks in advance.