Hmm, okay. I am struggling with this a bit.
In a previous playthrough, I had managed to get the scene where you can hfr fpvrapr xabjyrqtr gb vairfgvtngr Qentbpra'f qvfnccrnenapr, but since I didn't have the necessary hints, all of the possible responses were wrong (as outlined above). I had also managed to eraqrmibhf jvgu Infuxbg, but I'm not sure how to orpbzr n Yhzra orsber gur rcvybthr.
In my current playthrough, I did qvfpbirevat gung Mbenan vf n Abina pybar, but because I didn't get the chance to vairfgvtngr uvf qrngu, I got no closer to activating whatever's supposed to happen on turn 27. I got the opera, as usual.
What turn does the missing event happen in? What are the prerequisites? Is it affected by Mazarin or Esmarai? I'm not sure why that option didn't even come up.