fact is, the average witness isnt aware of 1919,proof for them is John 13:35 and Matthew 24:14
love among themselves
and the preaching wordlwide.
For a witness, these arguments cannot be easily overthrown. The reasons are :
1) love among themselves: It comes down to personal opinions and experiences. If you meet a happy dub (even though the happiness might be only on the surface) he is brainwashed into appreciating the world-wide brotherhood. The shunning policy, as a proof the the conditional state of this love, will not count, since it is presented as a loving way, unless he personally experienced it and thought about it deeper (most don't)
2) preaching, while it is obvious, that others participate in preaching in one way or the other, dubs are convinced that only they are recognized by it. Which is true, just look at the media, often portraying witnesses either being on the street or knocking one someones door.
Therefore, unless you have the time and energy to discuss these matters deeply, I would take another road.
Lets say, for arguments sake, the love shown and the preaching are indeed Gods will. So, does this mean, if an org does Gods will, they become automatically Gods org ? Lets say, the catholic church, they, even witnesses must admit, in some areas do Gods will. they distribute Gods word, they entertain charities etc...does this make them Gods org ?
So in order to be Gods org, an organisation must do everything according to Gods will ? So then, why were there changes with JW, regarding pagan traditions for example (christmas until the 20's ) ? Or matters like voting, civil service, blood fractions, organ transplants ?
Even if JW today do everything according to God swill, who really can determine this ? Only God himself, right ? So, were is his approval shown ? What certificate did he give ? O r what clear sign of his approval, that can only be traced to divine origin ?
How does he correct his org, as he did in the past ? In israel he sent his prophets to set matter straight, whom does he send today, since even JW admit the org is not perfect and need correction ? How is this done ?
Finally the fact that an org does everythign according to Gods will does not mean, God has an organisation ! This is a different matter. For example, he used organisations in the past (Isreal) who did often not things according to Gods will, and still were his org.