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Also quick question: Is the account that is being reported inaccurate in some way?
Since there is no account on EX, there is definitely no basis for a dispute, and it would not be a valid and you should not send the dispute to EX.
If the company verifies the information as correct (for EQ/TU) - then it will likely depress your FICO scores further, and yes, there is a chance that upon reinvestigation, the original company/account holder could report the updated/corrected/original (whatever is most appropriate) information to all three major credit bureaus (and possibly the secondaries), even if you don't dispute it with all three. This is why I ask the first question: is the account being reported inaccurate (credit repair agencies tend to dispute everything - valid or not).
I would add that there is nothing a credit repair agency can do for you that you cannot do for free, and most of us are happy to help you work through things.
They cannot dispute it in any other way than what you did already
If you feel like paying somebody to further ruin your credit, you're free to do so but i would really strongly advise you against it
Also, dispute same account multiple times without providing a proof of inaccurate reporting will lead CRA to deem those disputes as frivolous. They might have already
If you continue disputing without providing some kind of proof that what's on your CR requires correction, you can expect same outcome. At some point you need to let it go as you're hurting your credit more each time it comes back as validated.
It's totally up to you if you want to proceed and if you do, good luck.