Thank you for sharing the links. The first link contains some interesting information. If that information is correct, lounge costs are borne by the operating carrier (Cathay), and Cathay must purchase miles from the airline to which the flight is credited and then award those miles to the passenger.
According to the author, Scenario C is the most favourable outcome for Cathay.Fly Cathay → credit miles to Cathay → use Cathay lounge

• Ticket revenue stays with Cathay
• Mileage liability stays internal
• Lounge cost stays internal

Result: clean internal cashflow, maximum loyalty. Cathay “wins” on every dimension. That is best for Cathay

However, Cathay’s lounge process forced me into this choice:

• Either change the frequent-flyer number to Iberia to recognise Emerald status, or
• Lose First Class lounge access

I accepted the change:

• Cathay would still receive ticket revenue
• But Cathay would now need to purchase miles from Iberia and award those miles to me
• Lounge cost would still be borne by Cathay

Result: This would turn a financially optimal case into a financially worse case for Cathay and for the future. Is this outcome rational from Cathay’s own financial and loyalty perspective?