It would be helpful if you can give us the exact date of travel.

Usually "I" class is reserved for CX's deep discount Business Class seats. When it's a Buy 1 for 1 offer, the "I" class passenger doesn't get any miles.. "I" class is also used for the AsiaPass upgrade to Business Class.

However, "I" class is used in Asia for their deep discounted Business Class seats, and those tickets earn mileage.

CX's system has a way of weeding out which tickets can earn mileage or not.

But in any case, putting yourself on waitlist is the best you can do right now. I believe both tickets must hold confirmed seats before the tickets can be issued. No standby is permitted.