You asked "if the GB ever gave an explanation of HOW they were guided by the spirit." Like Elijah III, I do remember the explanation that members of the anointed write into Bethel about their understanding of scriptures and scriptural issues and that is how it can be said that the faithful and discreet slave provide the food although the books and articles are written by a comparative few. I'm not certain whether it was a Question from Readers or a public talk that I got this from...probably the former and most likely some time in the 1950s if I am to listen to that part of the brain where the memory is a vague shape in the mist.
I'm not entirely convinced by this explanation as my grandmother who was baptised in 1919 and believed herself to be of the remnant had a very deep but simple faith and would have considered it presumptuous to write in and tell them what her understanding was. And even had she done so she would not have "pulled rank" and said she was anointed so they had better listen to her ideas. Of course there were doubtless members of the remnant who did write in but I am just saying what my own direct observations are and wonder if that is not true of the majority.
"Beware the Jubjub bird, and shun the frumious Bandersnatch!" - Rev. Charles Dodgson