LOVE it so far, really helps build character.
I really like the idea of requiring bimbo magic, but I still would prefer if it remained optional... and yes, I'm aware of how counter-intuitive that is considering the subject matter, but it helps the player feel like it's their choice.
So maybe you CAN use bimbo magic at least once to relax her a bit to go along with the party idea, or your Charm needs to be high enough to convince her. Since it's much easier to just cast a spell and call it done, most players would go along with that anyway. This way, players are still compelled to use bimbo magic, but are in no way required to.
Alternatively, we could give the player another way in by starting Pinkie's S-link, but I feel that could overcomplicate this little proof of concept game a little too much.
Understand the wish to play it without the use of it, and sounds like raising the charm thing is a fine optional choice as well.
Thinking in just this single case it might be fine to pass it, just to keep it simple.
Like no matter how much bimbo energy she already has had before doesn't influence anything. what matters is the action of using it after the 6th S-link. so no matter if her bimbo stats were 1 or 10, it's just a required thing to do at this point so the story would could just progress as normal.