3:) Can you explain, using only the Bible, why we should believe that today's Governing Body of Jehovah's Witnesses is the sole channel of communication between God and mankind?
The Governing Body is not the sole channel of communication between God and mankind. God's word and guidance by God’s spirit as application to all Christians. The administrative position in Jehovah's Organization is simply to maintain orderliness as is any administrative position in a profit or non-profit organization.
In the formation, organization, and subsequent direction of the Christian congregation, the apostles occupied a primary position.
Now you are Christ’s body, and members individually. And God has set the respective ones in the congregation, first, apostles; second, prophets; third, teachers; then powerful works; then gifts of healings; helpful services, abilities to direct, different tongues. 1 Corinthians 12:27-28
And he gave some as apostles, some as prophets, some as evangelizers, some as shepherds and teachers, with a view to the readjustment of the holy ones, for ministerial work, for the building up of the body of the Christ. Ephesians 4:11-13
Although they were joined by others of the “older men” in such supervision, the administrative position formed a principal part of the expanding Christian congregation, and this body was recognized by the early Christians everywhere as a channel used by God to aid in rendering decisions and directing the affairs of congregations throughout the earth, not as a channel of communication that supersedes the Bible.
Now as they traveled on through the cities they would deliver to those there for observance the decrees that had been decided upon by the apostles and older men who were in Jerusalem. Therefore, indeed, the congregations continued to be made firm in the faith and to increase in number from day to day. Acts 16:4-5
This was possible for these men only because of the fulfillment of the promises about guidance by God’s spirit.
When the helper arrives that I will send you from the Father, the spirit of the truth, which proceeds from the Father, that one will bear witness about me; and you, in turn, are to bear witness, because you have been with me from when I began. John 15:26-27
Such help enabled them to recall Jesus’ instructions and teachings, to clarify points of doctrine, and to be progressively guided “into all the truth” revealed through them at that apostolic period.
But the helper, the holy spirit, which the Father will send in my name, that one will teach you all things and bring back to your minds all the things I told you. John 14:26
The same thing applies to all Christians, so when certain decisions are reached by the Governing Body the decisions are easy to reflect upon and understand.