I have no doubts that Russell sincerely believed his time table and the dates that he set. He was willing to admit that if he was in error then the time table could be off by as much as a century. But it is worthy to note that he did not abandon his firm belief that he and the other Bible Students were living in the time of the end. Nor did he abandon his belief of Christ's invisible presence.
On the other hand, I do doubt the sincerity of Rutherford. I think he had the devious mind of a shrewd lawyer. The failings of Russell's predictions were clearly evident by the time he took over the organization. His task was to keep some semblance of the teachings of Russell and, at the same time, try to explain away the errors. Since 1914 was a critical date in all Watchtower theology, he lifted the application of 1874 (Christ's presence) and put it on the date 1914. I also think that he came up with the idea of the return of the "ancient worthies" to distact whatever members were left in the organization after the defection and focus them on a new prediction. I have no doubt that this was a calculated move on his behalf.