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Backing up a step, a DV letter is allowed through the FDCPA. Unfortunately, the FDCPA applies to CAs only, and DV letters won't work for OCs. Early in my repair process, I didn't know what letter was for what and I too sent DV letters to OCs. Most do respond in kind as they did for you.
I've seen other posts mentioning credit protection plans. IMO, they are a scam. They really don't cover anything and there are so many legal hoops to jump through before coverage is activated. Do you happen to have a copy of the agreement? Definitely worth a look. Like with payment deferrals or payment plans, a credit is allowed to continue to mark you late even if on a plan. They can do so because the amount paid is less than the org. minimum amount set in the original CC agreement. I'd review the credit protection plan to see how it addresses reporting, if any.
If I had that I would send GW letters to both accounts asking for deletion if reporting negatively and I would include 2-3 sentences about the credit protection.
First Premier is difficult to work with. I would go through the protection plan people and address your concerns through them directly if that is even an option.