You have grounds for a complaint.
The severity of the damage and the insufficient repair means your car is not worth what you were told it was worth.
Especially with a 'certified used car' they are garanteeing it is good. But it was not.
If you like the car, then you can just skip the headaches, other than to ask for a rebate comparable to the hidden damage and its value reduction to the value of the car... from the dealer.
If you hate the car, get a lawyer.