"But why kill a leader that is anyway going to lose the next election?" - First, I repeat: revenge. Second, you forget about all the other passengers, 9 NATO's generals, leaders of every types of army in Poland, forcing to build the American Missile defense system in Poland, other important political officials. "why kill him in such a visible way?" - and why  Politkovskaya or Litvinenko or others killed in such a visible way? "and how to make the plane explode?" - a good question that has to be investigated. As it is known, the plane was "repaired" in Russia before, in Oleg Deripaska's facility. "and how to make sure nobody finds out" - again, like "why kill him in such a visible way?" - It's like other political murders in Russia - officially an accident, but who has to know - knows it and is terrorised and "warned" not to oppose. "why kill Kaczyński and not Saakashvili" - I don't know, ask Putin. 
 I don't know any opinion of any other "Aviation Safety Committee/Agency in the World". But, politically, who in the world, from all the most important countries, is suppose to care? Maybe Germany - deeply related to Russian officials with common interests ( eg. Nord Stream )? Maybe all the EU - same... Maybe USA with Democrats ruling and their "politics of reset" with Russia and, again, common interests in oil/gas production?