You take the gamble if you ticket an award ticket in lower class than what you are actually redeeming. As far as CX is concerned they only see that you were booked in Y. I would consider yourself lucky if they seated you in J.

The partner availability is a subset of AM availability. I would have checked using either Qantas or BA tool whether the award class would have been available to ticket and then called Alaska.

Earlier this year I flew MEL-HKG in CX C and was underwhelmed about the "experience".