You can safely ignore anything and everything from Trace. They are a powerless debt collector and can only try and intimidate the low-hanging fruit on the gullible tree into paying out of ignorance and fear.
Whilst a DSAR is OK, I would have advised you send a formal complaint about non-receipt of the original Notice to Keeper (NtK) and put them to strict proof of posting and asking for a copy of the original. A DSAR will get you a copy but likely without the required proof of posting. Service of the NtK is a rebuttable presumption. Whilst it is deemed to have been "given" two working days after it was issued, by rebutting that presumption, the burden of proof falls on them to prove posting or delivery.
You may want to check your V5C registration document. Often, the reason for not receiving the PCN is because after the last move, you may have failed to update the address with the DVLA. Updating a drivers licence does not automatically update the V5C.
When you receive the copy of the NtK (only the original, not any reminder, is relevant), show it to us.
