Given the severity of the damage I really think you should lawyer up if they demonstrate even the slightest hesitation in making you whole. If you go that route, be sure to press for pain and suffering due to the negligence of the officer who was obviously distracted and not safely operating the car. What would have happened to a citizen causing such a horrible accident? The same should apply to the officer involved. You need to be compensated for each of the following:
+ Diminished value to your vehicle (likely at least about $20-25K)
+ Loss of use of your vehicle for 4-5 months (Likely about $36K, based on per day rental of $300)
+ Repairs to your vehicle (likely about $30K)
+ Pain, suffering, and inconvenience caused by the negligent driver (at least $20K)
+ Provision to exclude future medical issues that might arise from any settlement.
Based on all this if I were you, I would absolutely not accept back your current car under any circumstance. They destroyed it, they should keep it. You should not have to go through the hassle of not having a car for months and consequently being in a less safe car than the one you paid for.
They should buy you a brand new Model S. If they refuse to do that, let loose a personal injury attorney on them. Perhaps with a large enough expense, they will have their officers better trained to pay attention to the road when they drive.