No, not if we've observed it. There are predicted historical events which can be shunted into the future by running away bravely, but if you've seen them happen, you can only change how long ago.
no, relativity allows for reference frames where the event hasn't happened yet, even though it has happened for you. there's no simultaneous "it has already happened" for all frames.
There is if the event is in the past or future light cone. If A sends a signal to B, and B receives it, there is no reference frame where that signal went backwards in time. (Unless you count exotic ones, like "particle with imaginary rest mass is zooming along at thrice the speed of light", but you can't accelerate to such a reference frame so I don't.)