Its a Fire Emblem-clone engine. There's a save file floating around with the full recollection room unlocked somewhere if its too much to bother with.
Recollection room is a catalogue of all the actual porn images/animations you interacted with. A save with access to a full one lets you just go look at the porn.
You should play new game and when you reach quest hit annabele with only "Player"
If you're weak enough that you do <10 damage (I've usually done it 9 point at a time) with your attacks she'll mock you about how weak you are when you attack and you'll eventually wear her down and get the scene.
You're not the only one. It seems to get locked up with certain scenes, and the only solution is to reset the game.
The white key is obtained by doing the main quest for chapter 5 where you fight Charlotte. After you complete it, you go back to the intermission screen and you can select the Royal Corridor sidequest to access the final battle.
There is a save with full recall room in the forum here, limit your search parameters to saves and search it, should pop up easy.
Keep healing items for the PC to tank the poison, and the actual kick off won't happen until you interact with the body, so take some time to position your guys. Just know the boss will insta kill everyone she needs to to get to the door and lock it, so leave her at the door, PC in the room, and surround her besides with the guys you need.
It doesn't have to be the MC who inspects "Roy", but do only have one character in the cell with "Roy" (preferably one with high Def so they take minimal damage from the arrows themselves), and have no one standing between the boss-to-be and the door. You can also unlock the cell door after it is closed with a key, once the boss is out of the way.
