"WD's implementation of AFP may be incomplete/lacking".
Or, it may be completely functional, because WD worked closely with Apple to make sure it worked. If you knew anything about how large companies work with partners, you'd know how logical this perspective is. It is to both their benefits to make sure things work properly. As opposed to your perspective, which is based on supposition and what "may" be.
"Your final comment of "should" is correct but at odds with your claim of "supported by Apple" "
Not at all. The pairing of WD devices with El Captian darn well should work. And, in my case, they did, up until I installed El Capitan. What other interpretation would you, in your robust and comprehensive experience, make?
You mock my experience in enterprise backup / restore. Tell me, when was the last time you worked in a data center with over 7,500 servers of all types to build out a consistent environment for backup and restore across them all? Oh, never? I see.