I want to lay out some personal opinions at the moment while facts are still coming out.
To start I believe that Turkey probably done screwed the pooch on this and should solely face Russian wrath for this. That does not include war, that being said I don't the NATO should be involved if it comes to war between Russia and Turkey. Turkey started it. again hopefully we stay the eff out of it.
1. Everything I have read all pretty much agrees that the Russian plane flew over Turkey on this mission. Turkey has made it very clear it won't tolerate this, which is its right. I don't think there is anything debatable in this point, but feel free to comment.
2. A couple options of how the scenario happened, all imo, and none of them are clean. All of them are based on my first point.
a) Turkish plane was shadowing Russian plane as it skirts the border. Once it crosses over Turkey the Turkish plane warns him, the warning part seems to be an agreed upon point, then shoots him while over Turkey. Plane falls inside Syria. IF, if if, this is the case this is the best case scenario for Turkey. however I see it as very unlikely simply due to the speeds they were likely flying. seems extreme that there was enough time to warn the Russian and then engage him over that little piece of Turkish land.
b) Turkish plane is shadowing Russian plane as it skirts the border. Once it crosses Turkish plane warns the Russian and engages while Russian plane over Syria. Seems more likely again due to speeds. In this case Turkey has some serious ass explaining to do and should face heavy penalties. But not war, Russian plane violated their territory on a "hot" mission.
c) about as unlikely, maybe a little more than a. Turkish plane is shadowing but warns and engages a Russian plane that never crosses the border. Warned and shot him before he got to the border. However where the plane comes down in Syria says to me it was unlikely that it happened before Turkish airspace. The flip side is also possible. Engaged after it had cleared Turkish airspace back into Syria. In this case Turkey can't explain things away and faces heavy penalties. this is obviously the worst situation for Turkey, and world peace.
these are the three scenarios which came to my mind. feel free to comment here.
3. I am willing to bet Syria and Turkey have some sort of understanding about airspace violation by either party on the other. especially with a war going on. It probably includes a buffer zone of some sort. If this does exist, and it seems like it should, I have no idea how Russia fits into this. Turkey had made it very clear that Turkey was a no fly zone for the Russians. If Russia didn't have some level of understanding with Turkey, they share some blame here, they violated Turkish airspace. If there was an agreement and Turkey violated that they carry all the blame. and there is plenty of middle ground where both sides are responsible with most of it falling on Turkey.
4. Unless the issue of air violations was brought before NATO before this instance by Turkey they can't make an Article 5 argument. If this is the first time it was brought before NATO, Turkey is the aggressor and can't call up the defensive alliance. If Turkey had brought up the issue of Russian planes being a reason to call in Article 5 before then Turkey could very well call up NATO. However I don't think this is the case. Turkey has said it won't tolerate these "incursions" and has talked to various parties, including Russia; but I don't know if it has been a formal motion brought to NATO. I forget which Article it is but each member state can call a vote/ruling, on if an instance falls under "defense" and it isn't clear cut. which in this case Turkey can't say this was clear cut defense against an act of aggression. So unless they called a committee before hand they are SOL, imo, on the NATO front.
5. How all parties treat this will tell a lot about the situation imo. too many options to list them all. if everyone steps down this is hopefully a one off situation that doesn't go anywhere. If one side pushes an issue its got an agenda. I think the most honest response would be for this to go in front of the UN. With this being Russia it can't be swept under the rug and Turkey will pay. However if Russia goes straight to war or keeps "buzzing" Turkish airspace they are pushing for a conflict and not a resolution to the matter.