So, you have 1 "penalty" for an "offence" from a council and 15 invoices from unregulated private companies. I really don't know why you are still referring to the as "fines" as it only serves to solidify Hertz's own lack of understanding.
How could you accumulate so many parking charges over such a short period of time?
This bit is crucial in the response you received:
Some of these will be sent to us for payment, which we will pay and recover from you by way of reimbursement. Alternatively, we may be required to provide your details to the relevant authority or other third party, who will contact you directly. Where we pass your details to a third party this will be in accordance with our Privacy Policy.
All the parking charges (PCNs) should have had liability transferred to you by Hertz according to PoFA section 13. Once that had been done, there would be no more liability to Hertz and it would have been up to the PPC to issue you with an NtH and you could have dealt with it.
If Hertz paid any of the NtKs (PCNs) they received from APCOA or MET, then they have breached the CRA. There was simply no reason for them to do so and if they did, they removed any possibility of you appealing the PCN.
As for the Admin charges, that is between you and Hertz.