Tom,
1)Keep the car.
2)Make a list of all the parts you need to fix the car locally.
3)Send the list to John and ask him to fill the order.
4)Do the work locally once you receive the parts.
5)Send John the bill for any work performed by a licensed mechanic.
Even if you have a professional do the repairs, I doubt it would cost $800.00. Hopefully, JPS will reimburse you for at least some of your out-of -pocket expenses.
If Gerd is right and John has figured out how to fix the leak then he can explain it in an email or over the phone.
Do not send the car back.