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Mail a DV to the new CA.
Wait on the response of the other CA.
Yes, you can DV the law office, as they apparently concede to being a debt collector by sending you dunning notice.
Whether you want to or should DV them is not absolute. It depends on what you are trying to do.
You sent a PFD, which a letter offering to negotiate the debt. The law office responded as an agent of the debt collector.
If you send them a DV, you automatically, based on the cease collection bar it imposes, preclude the law office from continuing their collection activities until such time as they provide verification. Is that what you want to do, since their role might facilitate the negotiations that you initiated?
Its not black and white. If the debt is not contested, a DV might do more harm than good.
My concern is that I already sent a PFD to the first collection service (American Accounts) that the law firm is claiming to represent. Does that mean they can't respond to my PFD?
If they still own the debt, they can legally negotiate its payment.
Having hired someone else to do it for them, they may just not want the hassle. The only way to find out is to try.