https://support.google.com/legal/troubleshooter/1114905
Proceed as if reporting infringement, and the option will appear to specify this is a counter report.
If the reporter chooses to continue, the next step is with the courts.
Right, because the DMCA takedown notice procedure as a whole doesn't apply to circumvention devices, only to content that is itself copyright infringement.
They don’t have to accept it-they can choose to ignore or reject it. However, if they choose to ignore/reject it, and the notifier then sues them, they may lose one of their legal defences.
Obviously they decide that most of the time complying is the legally less risky path, so most of the time they comply. However, if they get a DMCA request for a famous website like nytimes.com, they probably won’t action it.