It is hard to argue with support when they say your hardware is defective, especially if the warranty has run out.
I know you are suspicious of WD support and who wouldn’t be, but it is still possible that they are right and that you have some bad sectors on the HDD which is perhaps more than a few years old and that the bad bad sectors affected the ‘private user space’ and not the SMB or AFP public share of the MCH. The private user space is usually packed with small files and rewritten frequently and that is often the death knell for old HDDs, especially the kind of SMR (Shingled Magnetic Recording) drives that WD unfortunately used in the MCH. The use of SMR is the primarily reason that I preemptively replaced my MCH drive with a SSD eventhough I still had nearly 2 years of warranty left in most of MCHs that I have.
The only advice I have is before you take apart your MCH, test it on the only other OS that supports it - that is pull out your Windows laptop or borrow one and install WD Discovery on it and see if you can access the MCH that way.
