I have researched this and I can not find one decade in their history where they taught things that held true or didn’t have to be changed later because of it not being accurate.
As far as their preaching work, you can look at 1 Corinthians chapter 15 and it explains in detail what the good news is. JWs don’t preach the good news.
Point 2. The first three councils delt with the evolution of Christ finally saying he was both man and god part of a three personality. Yes thier are scriptures the seem to indicate jesus separate from his father, but I believe this is because the scriptures where written and changed over time to reflect the early Christians beliefs as they evolved over time. For example One can see changes about his birth and mother from gospel to gospel. Having a human mother was a later idea possible inserted after the council of Ephesus.