What a disgrace
I read thru the posts and what a disaster.
The goods purchased have obviously failed.
But they seem to be saying they havent failed because the inner rings are just loose and the plate can still be used. They are relying on the fair wear & tear principle (I am in Australia but so I only presume it is similar in the US - since my sons involvement with the sport my understanding is that a good bumper plate will last years of constant use if used correctly).
Instead of lining them up on the wall and not using them I wonder if it would not be more fruitful to instead use them more often so that the inner ring does separate which seems will eventually occur. If and when they do come adrift it doesnt sound as if they would be unsafe.
Even if you didnt do it with all the faulty plates I think it would strengthen any case you may have if there were at least a few that were broken right off. Then it would be a matter of, first they come loose and then they break meaning all the loose ones would break.
Your problem is that as a reputable purchaser you are dealing with an disreputable vendor.
A few months ago I bought my son a power cage and it was little more than pressed metal but fortunately it was 90 kgs lighter than the advertised price and I was able to return it.
I usually mind my own bees wax but the above was a thought that occurred to me as I read their disgusting letter to you in avoidance of being ethical.