I kept pushing back on the RMA dept (who tried to get me to let them destroy the backplane lol), reopened a support case, and one particular support rep (Paul B) has been helping escalate internally. The backplane finally got to a "Systems PM" who may have actually resolved the issue, meaning it was a firmware issue all along, and not a bad expander chip like the RMA team insisted on. Crossing my fingers here as they have to route the backplane back to the RMA dept who will then need to send it back to me.