When you think about it what Michael was doing in addition to making the family stronger covertly, was bringing "his people" into positions of power in anticipation of Vito's imminent death. That's generally what happens when any new CEO takes over a company. Clem and Tess were "Vito's men" and their loyalty was to him, not Michael. With the rise of Rocco and Neri, what was happening was Rocco got a promotion by Michael, and Neri was 100% Michael's man. That's why those were the two who went with Michael to Tahoe, and why Michael left Clem (succeded by Frankie) back in NY to run the olove oil business.