I'll take a pop at #2:

Once Michael whacked Sollozzo and McCluskey he had limited his options for the future and he had definitely kissed Senator or Governor Corleone good-bye. The dogs in the street knew that Michael had carried out the killing and even though Vito was able to buy the inaction of the authorities it would always be in Michael's background if he ran for public office. As you said in another post the principle of Omerta wasn't quite as water tight as the Mafia made out, so there would always be a Cicci or Pentangeli in the woodwork willing to go to the papers and say, “See that Corleone guy running for office, do you know what he did in 1946….”. Also in killing Sollozzo and McCluskey, Michael realised the power of the violence and/or the threat of violence. He had inherited Vito's ruthless streak and this meant vengeance for Sonny and Apollonia had to be taken. This meant taking on Barzini and Tattaglia. His path was set. He had to become Don Corleone.