Regarding Roth, he had two things going for him ... first the analysis that he would let the Cuban government do the dirty work is correct. It gave him a perfect alibi. Second is that Roth (who is based on Meyer Lansky) was basically the money man for all the mob. He had the ability to launder all the money that was coming in illicitly, so he had allies everywhee.
By knocking off Corleone, and using the Rosatos and other allies to seize their territories, Lansky could protect himself from any one who objected to the killing of Michael by giving that Don a slice of the pie....and besides, who in the mod would have objected to the Corleones being wiped out? After all that's the premise all three GF movies.