Even if the FBI didn’t want this case, there is the possibility of kidnapping, and that is a federal crime. They will have someone there observing, and will step in if they have to, even if if it reluctantly. They probably won’t have to, which is just fine because the Rangers can handle this without their help.
I doubt it. Given the glacial slowness of the Texas offices of the FBI, they probably just finished processing the paperwork and the FBI agent will be dispatched on Monday.
If the scenario is still ongoing, the FBI agent will be kept WAY WAY away from the actual location - say in the county sheriff’s office. And the FBI agent will probably like it that way.