Lars, you assumptions is curious. There are special procedures in place to deal with contingencies. Some deal with an N-1 situation, others deal with decompression (these are specifically designed to not run out of passenger oxygen). It is the raison d’etre for the HAMI route. The more direct routing for many of the flights being mentioned is L888, this posses such severe restrictions mostly due to insufficient oxygen for the passengers that at my airline we can only plan freighter aircraft along this route. The oxygen for crew is more than sufficient.
You last posts mentions that ETOPS was designed to be able to fly across oceans. It was not, it was an instrument to allow twin engined aircraft to operate more than 60 minutes flying time (in still air conditions) from an adequate aerodrome. It is true that it allowed twin engine aircraft to operate across the oceans, and was one of the drivers behind ETOPS. Maybe do a search on LROPS, both the Europeans and Americans are looking into this issue, and it will apply to any aircraft flying Long range operations, regardless of engine numbers.
As for a 1200 NM diversion on one engine, this is a highly unlikely scenario. At any point along the HAMI route there are enough alternates. Ranging from Astana, Almaty, and Urumqi on the west side, to Irkutsk, Lanzhou, Beijing, Xian, or Chengdu on the eastern side.
With a maximum take-off weight of around 297,500 kilograms on the Boeing 772ER (not unusual out of MNL) the restriction at DP2 of 285,000 kilograms is not a punitive restriction (enough fuel has been burned off by then). At least I have never seen it impact the planning for this flight.
You first question on why an airline would use a B772ER on a flight like MNL to AMS, the answer couldn’t be easier. It is the equipment that maximizes the revenue. MNL to AMS is a pretty tough stretch for a B747-400. My personal record on this flight is 14.15 hours. With these kinds of flight times the maximum payload on the 747 would be constrained. In other words; you would have to substitute fuel for payload. On a Boeing 777-300ER (also operating on this route on certain days) which carries one passenger less than the Boeing 747-400 at the airline we are talking about, the fuel burn difference is in the range of 25-30% between the two. Since 2/3 of all costs associated with our flight operations (in the order of two billion Euros, annually) concerns fuel you can imagine the cost advantages of using a Boeing 777-300ER over the Boeing 747-400.
Lastly oxygen supply cannot be easily expanded. It is not needed, as all contingency procedures cover this eventuality. Fitting bigger, or more oxygen bottles (both in use on the 747 and 777) would make both aircrafts empty weight a lot higher. You will haul this weight around on every flight (burning up expensive fuel). You will do this in order to service one route that we fly. Maybe this raises the question for you, why don’t they just use one airplane for this route with bigger bottles? Here you run into an issue that each airline has that operates these kind of sectors. In order to maintain a daily flight on this route you need at least two aircraft, since the cycle for a return flight is on the order of 28 hours. So by the time the aircraft would return to its home base the next daily flight will already have left. This means this aircraft will fly more and different (often shorter cycles) to be ready for its next long haul flight. Inherently meaning you are burning up expensive fuel hauling around these oxygen bottles on sectors where they aren’t needed.
I am very much on the inside at the airline we are talking about, and I can tell you that some very creative and smart people are always looking into all facets associated for maximizing cost/benefit. The Boeing 747 is in its last years with this airline (likely start of the phase out will be 2013/14). You are unlikely to see any new passenger aircraft with more than two engines at this airline, I am certainly not hearing of anything beyond 787’s or A350 or combination thereof. I hope I have answered some of your questions.