I think a good place to go to get some insight into this situation is to read Eric Hoffer's book, The True Believer. Short of doing this, try to remember the last time you tried to reason with a true believing Witness. It is almost impossible to penetrate their defenses. Now, the truest of believers are those who get promoted up through the ranks.
So, I think that yes, most of the top do believe it. If you have read Ray Franz's book, you will notice that it took him a long time before he started having doubts. It wasn't until he was assigned to do research and was instructed to follow wherever it took him, that things started to fall apart for him. Even then, he thought for awhile that he could change the system from within. He eventually realized that such an enveavor was fruitless.
There may be a few at the top who have some reservations. However, as victims of Groupthink, they would never express these to others. If they did express even a mild reservation in one of their meetings, they would either be ignored or ridiculed, so self censorship is the rule of the day in such situations.