The answers to the Lana/Evita posts are found in II, in the scene when Michael visits Pentangeli. He tells Frankie, "My father taught me many things in this room [the home's office]...keep your friends close but your enemies closer." He also said he wanted Roth "relaxed...to think our deal is still good..." Those were Vito's teachings and they applied to Carlo. He was an enemy, but killing him right away would have let Vito's enemies, still at war with him, know that he discovered who was behind Carlo's treachery. Better to make them think he was weak, and focus on getting Michael back to the US safely--then Michael could deal with revenge at the right time ("Revenge is a dish best eaten cold," Vito said in the novel).

As for things getting "rough": Michael needed Tom as his legitimate front man in Nevada, not as a wartime consgliere (which Tom wasn't, anyway).