If you are seeking a legal advice, this is a wrong place to do so. Moreover, I cannot help you there, in those parts of the world where I live you return the faulty goods to the guys you have bought from (i.e., the seller), not to the vendor. It is the seller's problem how to handle the process with the vendor or another supplier in the delivery chain, nothing for the customers (end users) to deal with. This RMA thing only seems to be commonplace in USA.
On the Windows diagnostics - totally irrelevant. The problem there is an OS-compatibility one. Their SeaTools (unless they have renamed it) diag tool does a low-level diagnostics
of the drive. It does not check OS compatibility at all, and it will not provide any useful information about defective-by-design firmware of the drive since the only thing it expects from the FW is to behave according to the manufacturer specs. If they are faulty by design, the tool will still report all OK in that department.
Before doing any other steps, would perhaps suggest to check with LKML folks about compatibility issues, clearly stating how which (horribly outdated) kernel version is QNAP using. They sure will have more insight on possible compatibility issues than me. I am no kernel guru. The only thing I could do is to download latest kernel from GIT and grep the sources for the drive model, to see if there are any "quirks" included for that. However, you can do the exact same thing yourself.