First off, Turkey shot down this plane for reasons important to Turkey. The plane may well have crossed into Turkey, though it sounds like it flew over a spit of land on an irregular boarder area, but the Turkish air force pursued it and shot it down over Syria, that is blatantly clear by where it crashed. Sure they "warned" the plane, for 5 whole minutes, but they obviously had the aircraft right there with the finger on the trigger and at the time of the shooting the plane would have been in Syria. Turkey was looking for an excuse an got one. The reason the plane was so easy to down was because the pilot didn't expect them to shoot. The reason for the attack is Turkey is trying to make it as difficult as possible for Russia to use it's air power, not because anyone legitimately thought Turkish forces or civilians were in serious danger. Had Russia actually been dropping bombs in Turkey, that would be a different matter and there would be undeniable proof.
If our top priority was to stop ISIS, we would support Assad and simply do our best to make the support conditional on reforms. He clearly only wants to stay in power and has already agreed to any number of reforms. This isn't to say that Assad is a good guy, but rather a statement that our so called allies in the area are pretty weak to non existent. Plus, at the end of the day I don't know that their priorities really match ours, all we really know about them is they aren't Assad and they aren't ISIS.
So why are we doing this? This is feeling more like a proxy war between the U.S. and Russia, and less like anyone actually cares about ISIS one way or the other. We aren't going to bomb our way to peace over there, making the war last longer is going to create more terrorists, not less. I don't know what they will call themselves, but I know we are just going to create more people with a grudge against the West, and I suppose Russia as well.