There seems to be some delays with Cathay Pacific A350.
Since I was on the first commercial Boeing 777-300ER flight many years ago (HKG to ICN), I just want to warn you that CX does not really do much for these inaugural flight events. They might make an exception since it is a completely new aircraft type, but don't expect much. About the inaugural dates, it will come last minute. Join the hkadb (Hong Kong Aviation Discussion Board) facebook group page, you might get more accurate and up to date information there.
If I am going to do an A350 inaugural flight, Singapore Airlines might be the better choice. SQ treats these inaugural flights as a high profile PR event. While CX might do an event, it might be limited to a press con and media. IF you want to stay with CX, you might have a better inaugural experience if you fly inaugural London Gatwick to Hong Kong flight. I know Gatwick airport has been trying to get more major full service airlines to return, so even if CX does not care much about it, Gatwick airport might makes it a bigger PR event.
Carfield