Thank you both for the insight. I don’t plan on taking many internal flights as I’ve heard about and have experienced (on the one internal flight I’ve done) the terrible delays and cancellations that plague internal flights in China. This means no weekends away around China for me if I can’t take the excellent train. This country’s loss.
So let me report my experience this weekend flying to Taipei. I checked in on line under MH but went to the business class check in to get a paper boarding pass. She reprinted it with my CX FF # and didn’t give me a lounge invitation saying I could use my MH gold card to get in. It worked. It also worked in the priority boarding queue.
I then checking in for the return flight on my phone while waiting to leave. It identified me as CX but still kept the advance seat reservation I made under MH some time ago. All good.
At Taipei i again went to the business class queue showing my MH gold card and asked for a paper boarding pass. She proactively changed me to MH to print a lounge invite and then changed me back to CX for the printed boarding pass. I don’t know if the lounge would have accepted my MH gold card instead but I suspect it would because it’s a Cathay own lounge at Taipei (and an excellent one at that).
So so I think this is it. I will reserve as MH enabling advance seat selection and then swap at check in in HK so my boarding passes reflect CX. Of course my MH status could all be over by end March and this process will be moot.