Perhaps the Klingons had help building the design? Here's an idea shortened up.
The Klingons figured the B'rel was aging too far and needed to be replaced by a new class of starship (same goes for K't'inga, but that's for another proper ship if possible) similar to the Defiant, a more-superior ship. The Klingon Empire were allies with the Federation, right? I figured it was a joint project and properly the class was named after the place where the first peace talks began between the Klingon Empire and the Federation, and the peace is going strong now. I feel it was properly named in that way, even though the ship was one of battle. I guess how the ship was run, I have no idea. But you could say the Starfleet markings on that ship with the Klingon markings indicate that this is the first of the class. Other ships of the class might not have Starfleet markings.
As for copying design? Well, it was said the B'rel was a design shared by the Romulans, as indicated by FASA or SFB, the brief alliance shared designs. This goes similar to the conception that the bird of prey (B'rel) in Star Trek 3 was originally going to be a Romulan ship stolen by Kruge. Well, carrying this concept further, we know that the Prometheus was attempted to be stolen and studied by the Tal'Shiar. Perhaps they got their hands (at least) on the designs of the Khitomer-class (if not an entire Khitomer-class ship)? Then perhaps they would've built a ship of their own (Nierrh) in similar specifications.
How's that for attempting to be creative enough to fill in some gaps?