You wrote "customary law". I'm not sure what you mean by that. Are your points 1 and 2 "customary" like a genarally followed voluntary courtesy, or, are they an actually ratified international Law, to which the Syrians are bound?
If the latter, could you please cite a source for this law? Thanks.
Also consider that Turkey is openly supporting anti-Assad forces with weapons and bases for the FSA, wanting Assad overthrown, even if they are not in an official state of war. In my other post I wrote this (underlined part relates to Turkey, due to aforementioned reason) :
Its also possible that perhaps the fighter was coming in too fast for the AA/SAM operator to contact HQ and get a response back in time to be effective if it was in fact, an attacking jet. Especially while Syrias military is on high alert due to tensions.