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How old is your bike/brakes? I have a Commencal which had the same problem. It was a few weeks out of warranty when I asked about it and they went ahead and warrantied it for me anyway. Either SRAM stood up for their product or Commencal ate it; I don't know. In either scenario, I'm extremely happy with the customer service. Try to get the brakes warrantied/recalled.

If I took my bike into a shop to work on something that was working somewhat normally before the warranty replacement, stopped working completely after they worked on it, then was told by said shop that I needed to replace it completely after their work, that shop would never get my business again. SRAM brakes are definitely annoying at times and I am having the same dead rear caliper as you are with my Code Rs, but that shop sounds like they're trying to back out of an issue that they helped complicate and are now putting the costs back...

That's a tough one Mate. There was the whole "Don't call it a recall." thing where they sent out thousands of replacement levers (Or perhaps hundreds of thousands.) a year or two ago. So one would think that those bad parts are finally out of the system by now. But that "Not rebounding quickly." exactly describes the original issue that they had. Perhaps your local shop still has bad parts in stock for your replacements, Or it could be that your levers do need replacing. Only the lawyers can tell at this point.