Makeit69
So I decided to test this and successfully managed to redo the entire Keigo/Reika/Takumi plotline without any issues.
To pull this off though I figured I'd have to reset ALL the quests that were connected with those characters to avoid any conflicts.
For those who want to do this, here's how I did it:
1) Look for your latest save file. You'll find it inside your main Mad Island folder. Go into "Mad Island_Data" folder, then "StreamingAssets" and finally "XML." That's where you'll find all your saves.
2) Backup the one you want to edit. It'll be titled something like SaveData07.xml. Just look for the highest number if you want your most recent save.
3) Once you did the backup, open the file in Notepad or Notepad++ or somesuch.
4) Use Ctrl-F to search for "QuestSave." That will bring you to the section we want to edit.
5) In this section you'll see ALL the main and sub quests in a format like this:
<questKey>Boss_Spider</questKey>
<progress>1</progress>
Now we need to find all the relevant quests and set their respective progress values to 0 (only change the number BTW, don't touch anything else).
These are all the ones I reset:
Main_Reika
Main_Takumi
Main_Takumi1
Main_TakumiFeed
Main_Reika1
Sub_Keigo
Sub_KeigoPendant
Main_Bridge
Sub_Prison
Note that Main_Bridge also needs to be reset to access the Prison sub quest (ie the one with Sally).
Some of these might already be at 0 for you depending how far you've gotten into the main plot, just make sure you set all of the other ones at 0 as well.
And yes, that's a lot of stuff to redo, but it actually goes pretty fast once you know what to do.
The only inconvenience I've noticed so far with redoing all those quests is that you end up with character dupes. Mainly Reika, Keigo (assuming you don't kill him), and Takumi.
I suspected this would happen and so moved my original ones outside of my base before I set off for the first event, just so I could keep track of which were which. I recommend doing this as well.
That said, it's not a huge issue, but it is a little bit weird having dupes of characters that should theoretically be unique LOL.
I suppose I could banish them--maybe even kill them, hmm. Might try that at some point.
EDIT: As per John's reply, note that you can reset the progress of those quests much more easily by using the in-game command /qp. Just use each of the quest names I listed above, one at a time, followed by zero, like so: "/qp Main_Reika 0" etc. Without the quotes, of course