A simple national security letter (NSL) without even needing to get a warrant and BOOM you can tap the server and get all info about the person running it.
Not if the server is paid for anonymously and you only connect to it over tor. That connection isn't through a hidden service and so isn't vulnerable to this attack.
A national security letter can not compel someone to tap a server for the government or allow the government to tap a server. A NSL can only request existing collected records. So for exampe a NSL could request any logs a service provider has regarding who paid for the server or any access logs they retain regarding the server. If they do not have any logs a NSL can't compel them to start collecting them. A NSL which requests actions or information outside of the scope allowed by law can be challenged in court.