Re point 3: I did the trip in November 2012. I was Y from SIN-CMB and F from CMB-DXB on two PNRs. I should emphasise that we were over 90 minutes behind schedule and that I had boarding passes for both sectors issued in SIN.
On arrival in CMB all through pax were asked to stay on board. After everyone getting off had deplaned I did the reverse walk of shame and went from the Y cabin up to the F cabin and spoke to the purser. She was happy for me to take my new seat and remain on board. Actually as I was the only pax in F she was happy for me to take any seat.
About five minutes prior to doors closing a very friendly gate agent came on board and checked my boarding pass. He'd received a message from SIN re my situation and he also (unprompted) assured me that my luggage was still in the hold and checked through to FRA (my destination after DXB).
I'm doing another of these ex-CMB trips over Easter but unfortunately no F on 349 this time - I'll have to make do with J. I've also got the same J seat selected from SIN-CMB with the intention of staying onboard. I won't be too disappointed if I have to get off though as then at least I'll be able to say I've set foot in Sri Lanka.