JD is likely correct.
There was a time when there weren't that many adjusters that would inspect the car prior to it being at the body shop.
If you had a good relationship with the owner they could write an estimate lower than what was actually needed. If the adjuster wasn't too savvy the work would then be approved.
After work commenced the body shop could then "discover" additional work that was needed after some disassembly and a "supplement" is called for.
The work has already begun and about 90% of the time there was almost no turning back and the insurance would regrettably have to pay the additional (supplement) in order to complete the job. That was an old trick that the insurance companies no longer fall for much anymore, as they're watching the bottom line for their investors.
That's why adjusters almost always look at a car before it gets into a shop and will write "heavy" if/when an older car is at/near a total loss. It's cheaper for them in the long run.
It sucks for the true car enthusiast.
If you make a claim the adjuster will likely total the car. Then your insurance will subrogate against the other policy-holders insurance seeking payment (was the other driver/car identified?) for the total loss.
You might have an opportunity to buy the car back and seek out some lower cost alternatives with used same color parts, there are plenty out there. Albeit the car will have a salvage title and only liability insurance can be carried on the car from that point onwards. Collision is off the table on cars with a salvage title.
Not having the car here to look at, the gap along the top of the left rear quarter panel is worrisome. Is it just the trunk panel pushed up and/or has the qtr. panel been bent? If the later it's a total for sure.
You should have a frank conversation with the adjuster if you want to keep the car? If so you should entertain a conversation about the use of aftermarket parts in order to keep the costs down. Who knows it might (?) bring it just below the total loss threshold?
I think you should get the estimate, weigh your options (you haven't made a claim yet, only an estimate) and make the hard choice; walk away or repair it with the above (?) caveats.
I feel and KNOW your pain bro.