2) I'd go with CX. The inflight service is a lot better, it will be more convenient for you to use the CX lounges at HKG (because the BA gates are a long way from The Wing), and when you arrive in London you can use the BA departure lounge at T1 before the BUD flight (albeit only the Business Class section if you are not Emerald - but that still gets you a shower, and you'll have had breakfast on the CX flight). The later flight is marginally preferable since the early flight often arrives at LHR before anything is working there (inter-terminal bus etc.).

4) On CX you get PJs on all long haul flights. You don't get them on the intra-Asia flights.

5) No - I reckon transatlantic AA F is not far short of BA's so long as you get a Flagship Suite flight. The AA FS is the best F seat in OneWorld IMO, and the food is not at all bad. On either airline you could get a good or a bad set of cabin crew. Take the EQMs.

6) Formally the answer is no. But people have reported that you may be able to get a supervisor to authorise it, particularly if it is just you (no guests).