“It would present a substantial challenge to Russia if there was a Turk excursion into Idlib. “
Putin has already hinted at using nuclear weapons. At the present moment, and until January 20th 2017 there is nothing to stop him from doing so.
Putin can’t afford to take mass casualties and he won’t allow it. Nor is he afraid of NATO while Obama is President. If Turkey moved into Syria and threatened the Russians Putin would take whatever action he deemed necessary to win. One bomb, and a couple of incinerated divisions, should do it.
But as for conventional arms, the Russians couldn’t afford to match the Turks at such a long distance; not for long anyway.
Also, the Turkish government is far from stable. If they take mass casualties the government might fall and Turkey with all its sophisticated weaponry, could become another Syria or Lebanon overnight.
NATO will not honor the mutual defense clause if Turkey pushes across an international border, so all that you said is likely true.
But the crazy jihadi bastards might do it anyway.