It is certainly likely that some do not believe it and are only after power, but I tend to believe the vast majority of Witnesses right up to the top of the organization do believe in it. Of course everyone has doubts to a degree and that can vary over time, but I think most of the governing body and others in leadership positions believe they are being guided by God and they are doing right.
Frankly I cannot understand when many on this forum take it as a "given" that Witness leaders somehow know they are wrong and are mendacious in their pro-Watchtower activities. As a psychological explanation for how these people act I find it altogether unconvincing.
Like over this recent change with the book study and the discussion about "what they are really up to". Well whatever "they are up to" I don't find it useful to proceed on the basis they mean harm or are purposely deceiving people. Whatever indeed they meant to achieve by effecting the change they must believe it somehow serves the interests of the "truth" and pleases God as they understand it to act as they have done.