Unique Cinematic Kill on Other Unity Targets
https://youtu.be/0b6YxsCHfzw
The scene that activates when you first enter the armor room underneath the cafe is supposed to happen only once in the game, but replaying memories is the exception. So of course to exploit this for a "scene assassination" on a target. I chose Sivert since he's the closest.
I left the setup out to save time, but the start of it (clearing the area of enemies) is similar to my eagle descent on Sivert.
https://www.youtube.com/watch?v=8N6HK5Botk0
You'll also want to kill any guards on the way to the cafe before leading Sivert there. Since combat with other NPCs causes Sivert to flee, armed civilians also need to be killed; berserk darts do the trick.
Now that Sivert is near the armor room it's easier to lose him and lure the target in the right spot using firecrackers. A high profile assassination is necessary because if a low profile assassination animation is cancelled (by the scene) before the target is dead, the assassination is cancelled.
The other targets this scene assassination should work on are Chrétien Lafreniére, Frédéric Rouille, Marie Lévesque and Louis-Michel Le Peletier. It won't work on the others either because of scripting or because of the area where those assassinations take place.
how does one even think to do this... 
“Force has no place where there is need of skill." Herodotus
Impressive stuff, Aurel. Thinking outside the (black) box xD
It's neat that the thought process for this is probably similar to triggering Invisible Kills from AC1, in the sense that you are being Aware of a cutscene trigger and playing with it deliberately.
Might as well mention a failed attempt at another unique kill. In the Peletier assassination, I moved him to the window scene start and tried assassinating him while starting the scene, with no luck. I couldn't get the scene to start after pressing 
, and if I tried starting the scene first, there was only a half second window of Arno moving towards the target for an assassination before the animation was cut off by the scene.