Okay, so my thoughts are basically along these lines:
I would probably start with an early choice which involves 3-4 branches in the beginning. That gives the reader something to choose and experience different paths according to their liking, but is still manageable. So, in this case, for example, the Mindslaver God shows up and presents the player with three options on means of control. Or, to stay closer to the setting, you get, say "10 points" to spend, but you don't spend them all at once, but only when the "God" presents you an option to choose.
Then, for example, you could choose between "sigil", "siren song" and "contract", and maybe one of those would indeed be cheaper, the other one more expensive, which makes the choice even more interesting.
In the next step, we could present the reader with another option on who to approach as its first victim. So let's say, there's Mary (shy but with hidden desires), Janet (self-confident) and Susan (rich, interested in music). Depending on which means of control you chose earlier, it would be easier or even impossible to get each girl under control. That would present a first very nice introduction into the whole topic.
Then, after you got your first girl under control, the God shows up again to grant you boons, so you can decide what to do with your girl OR you have to catch three girls first, before we get to the action part of having sex with them and so on.
The difficulty still lies in the complexity of branches we create here: 3 means of control on 3 girls is already 9 different story lines. Put 2 boons on top, and you end up with 18, etc. You could of course deliberatly decide to leave some paths open and concentrate on some others, but in my personal opinion, a story on CHYOA as a one-branch-story feels a bit lacklustre.
I am also a big fan of regrouping different story threads into one thread after a while, so that the whole thing becomes easier to manage. For example at the end of the day, we lead all branches back into one scene and start day 2 fresh (maybe with another girl, so that for the moment, the first one doesn't matter; you can still put in some smaller sentences referring to earlier decisions with if-conditions).
I agree with your idea that introducing a pact would probably add some spice and some more sense and purpose to the story. Problem again, in introducing several options, we'll get even more branches.
So maybe indeed, starting with one would be enough.
All that being said, there would be still enough possiblities to add other branches/choices later and add another mean on control, another pact, another girl, etc., once the story is set up and running.
