I don’t think it needs to be either/or, Tonus. I reckon everyone doubts to some degree, no matter what their belief or position. Rutherford had a lot of good reasons in his personal experience to suspect that he was being used by God. Consider his backstory of being imprisoned by the US government and being opposed within his own movement. Not only did he overcome all that, but by the end of his life the movement was thriving and expanding like never before. If you really put yourself in the mind the person, who wouldn’t believe you are blessed by God in that situation?
On the other hand, there is good evidence he didn’t treat his wife particularly well, was estranged from his son, fell out with many people, afforded himself luxuries in a time of poverty, not to mention suspicions about his womanising and alcoholism. Don’t these indicate cynicism rather than true belief? Possibly. Yet even biblical stories such as David combine true belief with shocking immoral behaviour. One apparently doesn’t exclude the other, especially for those in positions of power and wealth. We can judge their behaviour but it’s not the same as saying that he himself didn’t believe what he preached.
Strangely, I think a stronger case might be made for Knorr having doubts about the religion he led, because there is testimony from a few people that he didn’t believe the more eccentric doctrines and was far more interested in the technicalities of managing a publishing empire than in the niceties of theology.