As I found out the hard way myself (look for older posts of me asking similar questions), there usually isn't one, since other stuff that may have happened before/in between will have to be undone as well or your save will break from logic errors. While it may be possible in theory to find out the specific situation you're in, identify the flags and variables that would have to be changed and reset them (and lose quite a bit of progress as well), it will usually be easier to restart (and yes, I know how long that takes, done it multiple times by now) and this time make the correct choices. There is no "Cassidy_Investigation=FALSE" or similar easy console command to undo much of the stuff.
From what I understand not a lot has been written w/ Minami *yet*.
Saving on/before the event is really the only way right now. Gallery is really hard to update due to all of the variables that go into an event. save often, my current save is on page 29 but i was at 40 something a few restarts ago. It helps that you can now name your saves.
Well, there's the Ren'Py typical CTRL skipping through read text. And then there's the option of using multiple saves at critical points, so you can always go back to an earlier one where you didn't mess up, thus greatly increasing speed on reaching that point.
