Why can it be said with certainty that Christ’s original apostles were not at least part of the faithful and discreet slave?
Because, just like “offspring of vipers”, the second generation of the anointed “faithful and discreet slave” Governing Body with their overriding veto powers belittle what their “spiritual fathers“ once taught them.
During the annual meeting held on October 1, 1971, one of the speakers pointed out that the governing body of “the faithful and discreet slave” preceded the Watch Tower Bible and Tract Society of Pennsylvania by hundreds of years. (Matthew 24:45-47) A governing body was formed at Pentecost of 33 C.E., more than 18 centuries before the Pennsylvania corporation came into existence. At first, the governing body consisted of, not 7 men, but 12 apostles. Evidently, its number was enlarged later, for “the apostles and older men in Jerusalem” were taking the lead. — Acts 15:2.
(2001 The Watchtower, January 15, page 29).
By God, Holy Spirit is not of ambiguous nature and with certainty would take less than a century to reach the faithful.