Wonderful thread and I have learned so much about the accurate early history.
I was thinking of two more reasons "doctrine"changes.
1) public outcry on the part of the mostly asleep followers.
2) law suits
EXAMPLE of #1 is the bedroom policies of the '80s. People were getting scriptural divorces over oral sex! That caused mass confusion and anger. Pretty soon all that sank like a rock and was never mentioned again.
EXAMPLE of #2 This one is currently happening...all the law suits on child abuse have the leadership dancing. This is behind the distancing of the GB from the "local" Halls. They are putting space between the crazy local elders and the money the Society holds.
I guess the thought is if they back way off and leave the "infrastructure" in place (elder arrangement, donation arrangement) the average wittness wont know. The Circuit Overseers are the only legal connection anymore.
I would say inmodern day JW world, doctrine is determined by legalities.
Follow the money - predict the doctrine.