Now if we find more surfaces like this than maybe we have a hint
There are not too many "surfaces like this" on a 777. The landing gear doors maybe, the ailerons. Not too many of the parts are sandwich composites, not too many paerts are small and strong enough to basically stay in one piece, and not too many are likely to rip off in a controlled ditching.
Can the emerg pax oxy sys be deactivated so it won't fall down at the preset alt ?
Knowing that they do not fall down when you switch off all electrics... There should be a CB you can pull to prevent activation. You may however still release each of them them manually, if you have the right tool (the FA should have it), so definitely blocking them from a closed cockpit is not possible. Crew has separate portable oxygen supply which allows them to walk through the cabin and activate them one by one.
However, pax oxy is designed to allow a 20 Minute descend, not a 7 hrs cruise. I have no idea whether your wakeup time is in any way depending on the time you were unconscious due to hypoxia, or whether you will always wake up once you have enough oxygen supply.
Ethiopian did not do a controlled ditching. It hit the sea at high speed in a level attitude and broke up whilst the Capt fought with the hijacker.
It especially hit shallow water with submerged rocks. Many pieces of the wreakage did not sink, as it had ground contact.
I sometimes wonder how certain information is created, but that captain was joining a TV show the evening after the crash (I watched it that night) and explained in detail, that he was unable to estimate the altitude without clear reference, and tried to keep one wing low in order to notice when he really touches the water. The next day he was celebrated as an hero in the press. Strangely this version never made it into the official report, and the guy completely disappeared (contrary to Sully).
From a stricly logical point of view, if you take a lot of effort to let the aircraft disappear, you will probably go all the way to ditch it as intact as possible to minimize the risk of anything being found. If you do not care about that, why not crash it on the spot north of malaysia? So if the aircraft was deliberatly flown for 7 hours as far away from the shore as possible, a controlled ditching makes perfect sense to complete the plan.