I really find it hard to believe that anyone who studied the Bible without the Watchtower literature to "help" could end up with the same doctrines that the WT has. The Watchtower has said as much in their publications in the past.
You Can't Read The Bible Without The Watchtower!!!!
Seems to me you find it hard to believe what the bible really teaches. So what's new?
Humor aside...
I think it would not be so hard to believe, once one understands how it works.
Think of it this way...
A Muslim studies the Bible, and realizes that the Trinity doctrine is not taught in the Bible, but rather realizes that God created spirit beings, one of which is firstborn of all the others, and special like Joseph was to Jacob, and Isaac was to Abraham.
Likewise, miles away, an African Chief does the same... and a Buddhist, a Hindu, a Nazi Skinhead, a drug Lord, a soldier, a Catholic bishop, a Pentecostal pastor, a, , ...etc.
What do all of these have in common?
They all studied the Bible, despite being exposed to teachings to the contrary.
What did all of them see? The Watchtower? No.
They all saw the truth that was there in the pages of the Bible.
Does the Watchtower fit in there at all?
Yes, but not in revealing the truth to them. They saw the truth in the Bible.
The Watchtower aided them by pointing out truths - in the bible - in relation to what they saw in the Bible.
For example, the Watchtower aided all those folk above, with the scriptures that show the son is an angel - firstborn - and special. It is there in the scriptures all along, but hidden in the smog of the false doctrines.
In other words, the Watchtower showed them scriptures that otherwise were not shown to them, and or often were taken out of the context, or twisted to fit doctrines of men... like the Trinity, and Hellfire torment, and undying souls, etc.
When sincere people. who are searching for the truth, find it, they do not try to hold on to their doctrines for one reason or other - perhaps because they want to hold on to customs like Christmas and birthday celebrations, Mother's and Father's day, or they do not want to engage in the work the Lord assigned, because they prefer to relax, and not "take this thing so seriously"... etc.
Consider, it is Jehovah who sees the heart, and draws the one who has an honest heart. The person is also hungry for the truth, and humble.
John 6:44 ; Matthew 5:3
Though timid, or fearful, they are willing to obey the Lord's commission to preach the word, in favorable or troublesome season, from house to house, and door to door, to the most distant parts of the earth, among wolves.