Turnbull is right, like always. Michael needed Tom legitimate and he also needed to know that he could fully trust Tom. He coulden't risk losing Tom because he needed him to set up shop in Nevada. Also by keeping those secrets from him, when the shooting occurred, he knew he could absolutely trust Tom.

I don't think Michael felt that Tom was responsible for Sonny's death in any shape of the matter. I actually think that's a pretty silly statement. If Michael felt Tom was responsible for Sonny's death then he would have gotten rid of him long before Tahoe. Michael knew that Tom was closer to Sonny than anyone else and in the novel Michael expresses a twinge of jealousy at their relationship. Sonny and Tom allowed people in, in ways Michael never did. Michael was cold towards everyone, he never let people in, but that is what made him such a good Don.