I could be wrong but I am not certain Rome was crucial in the Council of Nicea. The emperor, yes. My understanding is that the Bishop of Rome was considered the first among equal. Most early Church fathers seem to come from outside Rome. Rome was much like NYC and DC, combined. It was important but not determinative. When I reach, certain ancient cities bishops seem even more important. Maybe it depended on the particular bishop and not the geographic location.
Rome became more important over time. Once unity was demanded by political figures, Rome became more dominant. The problem I see with explanations is the split between Catholics and Protestants colors the discussion. The evidence prob. cuts both ways. Catholics are certain Peter was the first pope. Others aren't certain Peter ever was in Rome. One thing is clear to me. No one was documenting this history or the records were lost.
I visited the catacombs under the Vatican. Early Christians were definitely there in a strong community. The contrast between the cathecombs which are not ad hoc burial places but rather crude nevertheless and the splendor upstairs in St. Peter's Basicila is striking.
I'd say Millerism and Adventists. Wikipedia discusses this. Witnesses and the groups from which they sprang believe the thousand year reign of Christ is very imminent and that they are restoring the only authentic version of Christianity. Christianity has not been authentic since almost the death and resurrection of Christ. I never noted a definite date when Christianity became fraudulent according to the Witnesses. I find it amazing that God would let humans live for countless generations without any guidance.
I keep forgetting the details for Miller. Sometime in the 1800s he predicted the end of the world. I've read accounts of what happened. Masses of people would ascend a mountain, selling all their posessions. Young children would be terrified of the end. Elation mounted up to midnight. (Can Christ only come at midnight?) The full day passed with no Christ. People were financially and emotionally destitute. He corrected his figures more times. No Christ. I don't know what innovations Russell made. Maybe none. He was wealthy and perhaps he just popularized it. My mom's family was active in Russell's time, She was a child. Rutherford was the one who truly created the Witnesses. Independent thinking was allowed among the Bible students.
I saw Russell era lit with my own eyes. It is full of occult symbols and crosses.
Hopefully, someone can explain. There are schisms within the Russell groups. Splinter groups emerged from the JWs. There is so much interesting history and we never learn it at the KH.