I wasn’t an elder, but my father and both grandfathers were.
This is how the arrangement works. Every congregation has an approved list of who has the “privilege” of giving public talks, and this list is regulated by the body of elders. The public speaker can either be an elder or a ministerial servant.
Each congregation has a “talk coordinator”. Talk coordinators will coordinate with other talk coordinators in order to find speakers to give the public talk on Sunday. Typically, they come from the general area: the circuit or a nearby circuit. So it would not be possible for a stranger to show up on Sunday and give a talk.
Even the content of the public talk does not originate with the one giving it. The content comes from Headquarters in New York. Basically, Watchtower gives dozens of outlines, and a speaker must select or be given one of those. While there is some room for variation, basically the speaker must deliver the points of the outline, albeit in their own words. Even circuit overseers are given outlines…although they have more freedom for variation.
The same holds true for circuit assemblies and district (or now regional) conventions. In fact, at a convention the speaker is given a manuscript and they must deliver it word for word.
There would never be any “fireside”. At least, not at the Kingdom Hall. There would only be the scheduled meetings Watchtower has directed. Extra meetings are a no no. There is a rare exception, but the visitor would have to be a missionary or someone from Bethel…it would not be a stranger. Perhaps use of the kingdom hall to watch some of the Society’s videos would be okay too.
Some sort of meeting outside the Kingdom Hall would be highly unlikely. I believe if a stranger showed up claiming expertise in something, inviting JWs to hear him speak, they would be very very suspicious, as well wary that this is an apostate. Independent meetings and study groups are a no no—even if pro JW all the way.
As the September 2007 Kingdom Ministry says:
“Does ‘the faithful and discreet slave’ endorse independent groups of Witnesses who meet together to engage in Scriptural research or debate? No, it does not …“the faithful and discreet slave” does not endorse any literature, meetings, or Web sites that are not produced or organized under its oversight."
Therefore, because of such a control structure from Watchtower headquarters, such a scenario you mentioned would be highly unlikely.