Can a pacemaker revive someone from a fatal arrhythmia?

Only if slow. A pacemaker will revive you from a dangerously slow heart rhythm. Most fatal arrhythmias however are due to ventricular arrhythmias that lead to cardiac arrest. These are very fast rhythms in which a pacemaker won't do anything for but an implantable defibrillator would help.
Fatal arrhythmia. A pacemaker/defibrillator can potentially revive, or cardiovert, a life threatening arrhythmia.
Probably not. Pacemakers can prevent bradycardia (slow pulse) or even heart block (no pulse). Some pacemakers can perform anti-tachycardic pacing, which can terminate some upper-chamber tachycardias. For the most part, however, tachycardias can not be treated by a pacemaker - and implantable cardioverter-defibrillator is required.
Depends. If what you have is purely a pacemaker that makes sure your heart beats at a certain rate per minute but is not able to detect a potentially dangerous abnormal heart rhythm then it won't help. Now if the device you have is a combination pacemaker and aicd (cardioverter-defibrillator) then this device is in theory capable of recognizing a dangerous heart rhythm and shocking you out of it.