i would say Give up

QF customer service are great in ORC (original routing.credit) done it many times but you are trying to credit to CX let alone on QF ticket stock

further you said "waiting for refund" above - you are asking for fare difference between C and Y and you flew Y so you were essentially ticketed for Y afterall.

further i think there are some T&C that states class u fly in is the credit not your ticket class. Read those legal jargon

finally my experience of Cathay asiamiles/club agents are outsourced unempowered agents following the rulebook. You have better luck writing to Customer Service of CX Headoffice and ask if they can help. Cathay Pacific Loyalty donkeys will not do much