Great thread. And a few comments/corrections
Regarding checking in baggage. I have had multiple successful through checkins on a LHR (BA) HKG, HKG (UA) SGN (and return) on seperate tickets. According to UA customer service there is an "luggage agreement" with BA and UA in HKG allowing for passenger to check through luggage even on separate tickets. It is my experience that most check in agents try to accommodate your requests
Re. Shareholder discounts. From 2010 S/H discounts were permitted on travels originating outside of the UK as well. The current conditions reflects this as well.