It would seem to me to be very easy to get a pilot to deliberately crash a plane if one were willing to be brutal enough. KGB/Mafia are certainly not shy about such things. 

All they'd have to do is show a pilot photos through rifle scopes of the pilot's loved ones. Then let him know he is already dead whatever he decides, but that he can save them by complying. He would then be told that must crash the plane or they would die. Of course he would do it, knowing that he was already dead or would want to be if he caused the other's deaths. A simple exercise in motivation. 

No witnesses, no evidence. Just unanswered questions that the Russian investigators already know should not be looked into too closely - after all it's Russia and the dead Polish guy was an adversary of Putin, so...a smart investigator knows what to do if they want to live. 

Maybe a similar thing in this case. For trained, well financed operatives it would be a piece of cake. 

That's exactly how Putin/FSB/SVR and the Russian Mafia (all the same thing) roll...they don't just /maim/slaughter/torture people because they have to, they do those things because they enjoy them. 

That is one of the things one does not joke about in Russian society - the brutality of the Mafia/government.