Jehovah used imperfect people to do His will yes that is true but did imperfect people DECIDE what His will was and change it back & forth?
I understand that men on the GB are imperfect so may think a wrong or thought or act unchristian by accident but whilst the men are imperfect IF the organisation is indeed Spirit Directed and these men are under the Holy Spirit then NOTHING they would produce as an organisation would ever be wrong.
The simple fact is every JW has to rationalize that either the Holy Spirit has got it wrong in the past (er no!) OR that the GB have produced teachings NOT from God...it is one or the other...being imperfect does not exempt them from the fact they have uttered wrong teachings whilst claming to be spirit directed...you can't have both!
If you read Don Cameron's book Captives to a Concept you will see how easily the idea that the WT/GB are God's Organisation on earth falls down...