1) Jump to a specific time (game time or machine time)?
Nope; you gotta fast-forward to the point you're interested in.
2)Rewind?
Nope; you gotta restart and fast-forward to the point you're interested in.
3)Fast-forward more than 8x?
Nope; that's maximum. In practice, though, very few systems would actually be able to handle higher than 8x once fleets get large.
4)Convert the recorded game file to a video or some other format which allows any of the previous?
You can always try recording the screen and creating a video that way.
The way replays work in Sins (and most games) is that they record the initial state of the game, the random seed, and then an exact list of every action ever made by the human player, timed down to the millisecond. Basically, if you put exactly the same inputs into the game at exactly the same time, the game plays out exactly the same way.