There is never any legal obligation for the Keeper to reveal the identity of the driver. If anyone told you otherwise they are either mendacious or simply intellectually malnourished.
So, you received a Parking Charge Notice (PCN) a few years ago. You appealed to the operator and subsequently complained to the IPC. Did you also appeal to the IAS?
As for the debt collector letter from Trace, you can ignore that and either use it as kindling or to line the bottom of a litter tray. Never, ever, ever, try and communicate with a powerless debt collector. They cannot do anything.
Have you changed address at any time since you received the PCN and appealed? If so, did you send a Data Rectification Notice to the operator informing them of your new address for service and for them to erase the old address?