The thing I worry about is that if we talk to Elle it may preclude a more daring approach. Attention would be on us at that point.
My most daring suggestion would be this:
Our goal would be to subtly corrupt their ritual in a way that makes it backfire.
Mix paint and water of forgiveness. Then go around adding "Touch ups" to backgrounds props and costumes. Be
very subtle. Sacrifice into them our emotions regarding Elle.
Once we are done we leave. Run as far as we can.
We can't win a contest of power, so we have to use their own power against them. The ideal outcome would be that at some critical moment of the performance it turns against them. "Taint" it with ideas of forgiveness and humanity. Anyone beyond saving is banished or destroyed, and anyone with enough humanity left is saved and cleansed.
Play this like we are the villain planting a seed of corruption. Not the hero. Magic can go disastrously wrong if there is a mistake. This time we weaponize it against them.
The risks are twofold. We can either get caught or we can bungle the magic. Both could spell disaster here in the heart of their power. But if everything goes right we could win.
I like the first part, but not the second. We don't have to vote for the final step in a plan until after carrying out the first post and getting feedback.
Let's talk with Elle, get some info how her mental state, and then go from there.
Maybe she's cognizant enough to have a feast of isis in her dressing room?
Maybe she's got her own plan? Maybe we come back tomorrow with some accelerants, break a binding on her, and flee while the building burns down?
Ah, we don't have time for that. There is no "Tomorrow". The play is starting soon.
... Though come to think of it, did we only just barely get the ticket in time? If we could have arrived a day before or something to set up a trap that would have been better.