Most of the questions you ask are answered in a somewhat older thread:
http://www.jehovahs-witness.net/watchtower/beliefs/192766/1/Was-There-REALLY-a-First-Century-Governing-Body
To answer some of your questions directly:
1) Basically in the 1st century every congregation was its own little group without much oversight, just a local group of older men. Jesus never organized a religion. Paul was not a travelling overseer, but a missionary, ordained by Holy Spirit and not by men (as you wrote yourself - a contextual reading of how Paul was called to serve is telling - see 4+5).
2) No, he did not. Peter was used by Jesus to unlock christianity for Samaritans and the nations. also see answer 1.
3) yes, as described in answer 1)
4+5) Let us read the verses: "Now in Antioch there were prophets and teachers in the local congregation, Bar′na·bas as well as Sym′e·on who was called Ni′ger, and Lucius of Cy·re′ne, and Man′a·en who was educated with Herod the district ruler, and Saul. 2 As they were publicly ministering to Jehovah and fasting, the holy spirit said: “Of all persons set Bar′na·bas and Saul apart for me for the work to which I have called them."3 Then they fasted and prayed and laid their hands upon them and let them go.
The Holy spirit decided to send Barnabas and Saul, and the 3 others (Symeon, Lucius and Manaen) who were with them, let them go. They were not sent by the congregation, but by the Holy Spirit. Not much room for interpretation here. Furthermore, the words of the holy spirit was directed specifically to the 5 prophets and teachers. "They" clearly refers only to these 5, and not to the congregation.
The rest is answered well enough in other threads, as the one I mentioned above.
For the following sentence, you did not provide any basis: "So what we have described in Acts is a doctrinal dispute that is referred to a central body for a decision. Then the decision is conveyed to the believers who assemble in various congregations."
The truth is: the doctrinal dispute is referred to the local congregation from where the problem started.
Many scriptures in the bible indicate there was no central authority in the christian congregations. To be honest, if they were indeed individually guided by Holy Spirit, there was no need for extra guidance. Revelation 2 and 3 point out that Jesus is guiding the congregations. If there was a central body, would he not have said something about it, like: you are following their lead well? or alike, but he did not.
Nowadays, would you take your disputes to the next meeting of the governing body, and would tell them they are the originator of the problems in your congregation? That is exactly what you are saying that Paul and Barnabas did. Very unlikely.
Would you consider it normal to tell the pope off in front of others because he is not having lunch with non-catholics? Because that is what Paul did with Peter.
Some thoughts maybe worth your considerations.
Hoffnung