The car and lease agreement are your responsibility. You would have been aware of the risk when signing up to the agreement.
If you try to withdraw from the agreement, I suspect that there will be a penalty involved. This would involve giving the car back to Mercedes and settling the relevant amount. I presume this would depend on the condition of the car and the mileage. If you could not afford to pay Mercedes the relevant amount, then your credit record/rating would be affected.
The other option, if you don't trust your dad with the payments, is to make your dad set up a standard order to your account, to pay you the monthly payment say 15 days ahead of the date that Mercedes need the payment. Then for you to have the direct debit or standing order direct to Mercedes, If you dad fails to pay the amount on the given date, then you can contact him to make sure he transfers the money to you. Atleast this way, you have some control over the payments.
In regard to your dad forging a signature on a delivery notice, I am not sure about this. Are you prepared for you or Mercedes to report this to the Police and for him to face the consequences of his actions ? I am not sure what he signed and what he could be charged for. I suspect that some offence has been committed.