Being in violation of the FDCPA, of which failure to have sent dunning notice within 5 days of reporting to a CRA is certainly one, is not an issue of credit reporting.
It is not a basis for disputing their credit reporting under the FCRA, and not a basis for CR deletion.
You can pursue it administratively by filing a complaint with the FTC or by bringing civil action against them. It is a violation of their debt collection practices, and is handled separate from your credit reporting issues. The FTC does take legal action on individual consumer complaints, which is one reason why debt collectors treat FDCPA violations kinda cavalierly. Not an easy or highly productive row to hoe.
As for DOFD issues, the first thing I would check is, if the OC ever reported their account to the CRA, they were required to have reported the DOFD on their account to the CRA within 90 days of referral of the debt for collection. Only the OC can provide an actual DOFD, and if they ever reported one, it trumps any reporting done by a debt collector. If the OC has reported a DOFD, the debt collector is required under FCRA 623(a)(5) to report that same date.
A fairly easy way to nail down DOFD, and prove illegal reporting of a later DOFD.
