One very difficult argument that I haven’t yet overcome is that if you believe (as JWs do) that a worldwide preaching work needs to be undertaken , then how is it possible to do this without any form of structure or organised body? Any argument that God didn’t use a similar organisation in the past is irrelevant as no similar work on such a scale has been undertaken in the past (as Tom Cabeen acknowledges).
The issuse is not whether "any form" of "structure or organized body" is required, but whether a formal earthly, "visible organization" led hierarchically by a central "governing body" is stipulated or required. The Holy Spirit unites all believers worldwide into an living organism regardless of formal organizational memberships.
The bible instructions to meet together , to speak in unity , to have “love among yourselves” , appoint elders and others in positions of direction and , above all , the command to preach would seem to strongly infer , indeed demand , an administrative structure.
Christians have always met together with or without formal organizations. Indeed Jesus said: "For where two or three are gathered together in my name, there am I in the midst of them." Matthew 18:20
To "speak in unity" requires being led of the Spirit and the scriptures not a formal organization. However to speak in uniformity (a WT requirement) does require a formal organization, thus they emphasis it.
There is no requirement for a WT like organization with its "administrative structure" in order for for Christian believers to have "love among yourselves". I personally am friends and love other believers who attend other Christain denominations or fellowships, and yet our bond is one of love in the Spirit. What kind of true love requires an "administrative structure" to be in effect?
The apoinment of elders in Acts 14 was done by Paul and Barnabus without any specific formal consulatation of a central organized body.
The command to preach was given by Jesus to all Christians, as well as the Holy Spirit and needs no single administrative structure. Indeed Paul took pains to state that he did not confer with "flesh and blood" nor go "up to Jerusalem" to the other apostles before commensing his preaching work.
Galatians Chapter 1
15: But when it pleased God, who separated me from my mother's womb, and called me by his grace,
16: To reveal his Son in me, that I might preach him among the heathen; immediately I conferred not with flesh and blood:
17: Neither went I up to Jerusalem to them which were apostles before me; but I went into Arabia, and returned again unto Damascus.