I would love to have the EFF chime in, but there are some protections for you as a host under the Online Copyright Infringement Liability Limitation Act (OCILLA) - or safe harbor provision in the USA.
As to how that has been tested legally on federated content, I don’t know. Perhaps another elder of the internet can tell me how Usenet servers handle it.
What are you implying here? That @gabe should never have bothered with running a server? What about the server you are connected to right now? Should they shut down because of what may travel across it?
No.
They’re protected under the same rules as somebody running a WiFi hotspot at a coffee shop. As long as they are doing everything within reason to be a good steward of their local network (which is what Gabe is doing) then they are protected.