With regard to the broken control horn on the flaperon, it may not have failed with a fore/aft load but with a side load, in which case it would be much weaker due to the much narrower section in this direction. This could have been in the crash, or it could have been after reaching Reunion. For instance, by getting wedged between a couple of rocks on a reef and a wave applying the breaking force. A study of the failure mode and the extent of corrosion on the exposed face should answer this.
I'm surprised that the damage to the trailing edge is being discussed as if this must have occurred in the initial contact with the sea. As this side of the flaperon is thin, it has very little buoyancy, so would have been hanging down in the water like a keel and would have been dragged across any rocks, stones or coral as it washed ashore.