Oh well,

Latest is that:

"Since you have never earned any miles with CX, and it is a free card issued with an Amex, then you don't get Main Cabin Extra."

Obviously, when we check in we can show the card and get free baggage, but I really cannot believe the hassle of something very minor.

Worst is, now we have ticked the Yes we want to pay for MCE, we can't get rid of it from the held booking without phoning the call centre.... again!