If somebody could add the option of negative arguments (for time reversal) in set-time-compression sexp..... 
Wouldn't help, because the engine does not keep track of objects in a way that would make this work. For simple stuff, like debris and unguided primaries or secondaries, this would be sufficient, but objects with more dynamic flight characteristics would behave completely wrong.
At some point during FS retail's development (waaaaaaaay back in 2000), :V: planned a "Replay theater" kind of feature, but it was never completed, and only existed as stubs and commented out code in the source code release. If they had completed this, it might have been used as basis for a time reversal feature, but as it is, we cut it out of the FSO codebase a loooong time ago.