Yes this is an interesting topic. I'm not of the opinion that the early church was in a total mess until 325AD, because those who knew Jesus were clear on his teachings and were the core individuals who taught and set up small groups. The apostle Paul didn't meet Jesus but did know those who did. He formed a huge number of groups during his missionary journeys and we know all of his teachings as recorded in the numerous letters he wrote which are now part of the NT. These are early works, and combined with records from those we call "early church fathers" we see consistency in belief, despite the splinter groups who were teaching heresies.
It isn't true, in my opinion, that 325 marked a huge apostasy, that is something Watchtower have jumped onto. I'd agree that the eventual formation of the Roman Catholic Church as we see it today *was* a corruption, but at Nicea the basics of the Christian belief system were simply recorded in order to address the issues of ambiguity.
On the resurrection, there is a lot of information out there, but this simple article gives a decent explanation. https://carm.org/did-jesus-rise-dead