It’s fairly easy to see why Obama didn’t fire Comey after his late October stunt, even though Comey clearly deserved it. There was already a lot of talk of how Obama was using his office to throw the election for Hillary, and that would have just added fuel to the fire.
And yes — Trump had the legal right to fire Comey for any reason. But he’s breaking the law if he fired Comey in order to stop the FBI investigation of him and his campaign team (and Trump did admit that the Russia matter was a factor in the firing in his interview with Lester Holt). That’s called obstruction of justice, and that’s a crime and an impeachable offense.
And do you really expect to be told in detail what information was leaked by Trump to Russia? It’s classified information. At the highest level. But if you’re looking for evidence that it was actually leaked, how about Trump’s word? He admitted it.