Yeah all of that is conspiracy theories. The "lead investigator" you refer to had his allegations taken down by Fox News because they "didn't represent the high standards of news" and he was told to stand down because of the threat of legal action by Rich's family.

I'd note that you fail to cite sources, but here's plenty explaining the conspiracy theory in depth.

https://www.npr.org/2017/05/17/528804792/unproved-claims-ree...

https://www.snopes.com/news/2017/05/25/seth-rich-conspiracy-...

https://www.snopes.com/news/2018/06/01/fox-news-seth-rich-co...