When I started my playthrough I didn't know if it was worth it accepting to have the cheatbook in my inventory, but I'm glad I did. I never use it in my main playthrough, but when there's an important choice I sometimes find myself trying to see what happens if you pick the "other" choice. When doing so, I try to speedrun the scenes I miss by manipulating the stats. Pretty useful if not abused.
Anyway, Kate is the best.
I just started to corrupt Natasha so I still don't really know what you're refering too. As for Marcus I'm even more lost ahah. I know he's her dad but where I am he's considered dead