a. By that point it seems he cut contact to Ola.

b. Michael was so insulated in Nevada by then they had to trick the dumb brother into feeding them info. That says something

c. Because then he would have for sure give it away that Fredo was the one that more than likely gave things away because only him and Roth knew he would be coming with that cash. Then Michael probably goes to war over his brother regardless and Roth can't have that.