They are not required to "prove" the accuracy, or support their finding with documentary evidence.
By "they" I take it the the CRA doesn't have to prove the accuracy or provide proof, but the CA would, right?
The furnisher is required to conduct a reasonable investigation, and report their finding to the CRA.
The CRA is then required to complete its reinvestigation, with the finding provided by the furnisher being part of the basis for their reinvestigation.
It is presumed that both the furnisher and CRA conducted a reasonable investigation, and neither is required to provide documentary proof to support their finding.
Stated differently, the dispute process is a review procedure, not a legal process that mandates support by documentary evidence.
Any "proofs" provided by a furnisher or CRA would not necessarily comprise all relevant evidence, and be one-sided and subject to contest by the other party as to its showing and/of completeness.
Absent a full legal discovery process, such as provided in a civil action, documentation is not required in an administrative dispute process.
I disputed a utility bill once with the Public Utilities Commission (Public Service Commission in NY). It wasn't an unpaid final bill, but continued charging after I was gone and a new tennant moved in. I claimed I had requested cancellation and provided my dates and the landlord's contact info. I think they lean towards defending the public, so it's worth a shot. If they rule in your favor, you could take that to the CRA.
Regarding my unpaid collection utility, the utility was for my mothers bill and when we moved her to assisted living I put the gas utility in my name until house was sold.. I sent a goodwill letter to the utility company to ask them to request collection to be removed by the CA. The utility company said there wasn't anything they could do about it until it was paid. I called the CA and they said they would have the head honcho look at it and get back to me. I figured I would just let it age off next spring but one day I looked at my reports and it was removed on all 3 bureaus. I don't know if it was because the utility company requested the CA to remove it because of the goodwill letter or if the CA did it per my request over the phone.You just never know how effective your efforts will be unless you try. You miss 100% of the shots you don't try, right...
I would like to suggest that if you tried all other avenues to get rid of collection and before you sue them, maybe try a complaint through the CFPB. I was successful in getting a collection that wasn't mine removed when the CA wouldn't answer my request for validation and other communication wasn't successful. It was real easy to do on their website. 15 days after I submitted the complaint the CA compliance department had sent the CFPB a complete history of the account as they saw it and said as a goodwill gesture that they would delete the account. Well worth trying for those who have tried everything else. It gets the collectors attention.