This list gets downloaded from whatever server you configure and is then local to your machine. Editing it would only affect what you see on your own client.
> After a time I got unhappy about this and went to see if I could determine a physical hosting location of the content.
Usenet is a distributed system - every server that didn't filter the group was hosting the content. They were typically viewed as common carriers.
(It sounds like you were tracing your own ISP's server?)
On that note some from upstate New York may remember that the attorney general tried to garner votes by raiding and seizing servers from a few local ISPs (Dreamscape, RIP), which in the end didn't work out.
> I wasn't willing to reach out to the FBI blindly and couldn't locate anyone who know anyone in the bureau.
The FBI was quite well aware of the content on Usenet and very adept at navigating it in its heydey. They used it in a number of investigations to track down producers including breaking into trusted rings that communicated with PGP encrypted communications on Usenet. You wouldn't have informed them of anything they didn't know.