good questions about their appointment.
When it comes to the appointment of elders by holy spirit, the explanation is something like this: The holy spirit outlined the requirements in the inspired scriptures for an elder, and if a brother fulfills them and is appointed, it is like holy spirit did it.
Now how would they argue regarding governing body? There are no requirements in the scriptures, its not even mentioned. And regarding the slave, it was only mentioned, that he is faithful and discreet and distributes spiritual food. But this he only can do, after his appointment. So how do new members qualify then ? And how is Jehovah (or holy spirit or whatever) involved?