FISA warrants are rarely denied because the intelligence agencies submit drafts to the court who tell them what is needed to get approval. Hence they never submit a warrant application that they aren't already assured will be approved. Anything insufficient is withdrawn before judgment.
Convenient, and perhaps legitimate, but not provably so. Courts are supposed to be adversarial, that's why we have prosecution vs. defense or grand jury. Judges are meant to be impartial arbiters. In the FISA system, judges play a dual role as both arbiter and defense, which is a conflict of interest.
It'd be a real court if there were a sort of public defender, and the judges simply mediated between both sides. You could see it working in the warrants rejected.
It is provably so. Namely if this is a serious interest I suggest you write to the House or Senate Select Permanent Committee on Intelligence [1] [2] and request documentation of such.
In fact the court is adversarial, though not like you seem to be envisioning. There are many courts in the US, in fact most where no litigation happens because it's inappropriate for the task. So the idea that they are playing "both sides" is a non sequitur.
The role of the court is to ensure that requests are legal and there is sufficient evidence to pursue the task as requested. It's exactly the same as when a judge issues a warrant.