If you step back a bit, you have to question Michael's statement that Tom was out, in part, because of his need to have Tom be "legitimate" for the move to Nevada:
If Tom were expected to take part in one or more of the hits during The Great Massacre of 1955, there'd be a risk of his being arrested or recognized, which would compromise his role in Nevada. But neither Michael nor his father had ever used Tom as muscle for the family. At the same time, Tom was still a full-fledged member of the Corleone Family, so any notoriety that stuck to the family as a result of the massacre (as Connie said, "Read the papers, read the papers") stuck to him.
IMO, if Michael really wanted Tom to be "legit" for Nevada, Tom would have quietly left the family months before the massacre to set up a law office in Nevada. He might even have taken on clients other than the family, to establish himself as an "independent" lawyer. He would have found a way to be someplace public and prominent during the massacre.