Account records are, of course, important in establishing actual DOFD, but just as important is establishing reported DOFD.  Two different things.

If the OC was reporting to the CRA at the time of their collection referral, they were required to have provided the DOFD to the CRA.  Any debt collector is required to report the DOFD to the CRA within 90 days of reporting their collection.  So reported dates are in your credit file, and are available apart from any issues of account records.

Two dates trump any issue of reported DOFD.

One, if the OC ever reported a DOFD, that is end game.  The debt collector must report the same date as any reported by an OC.

Two, the debt collector can never report a DOFD that is later than the date they received collection authority, which is usually the date-opened of their collection.

Are either of these two dates established in your credit file?