Second comment following up to the first I had made previously. For an interesting update, I have, in fact, found where the Heart Key is. It is in the second to last room, where you talk with Adrien before going on to murder Elaine. Interestingly, there isn't any such key in that room. How did I know? I checked the game files and, after a bit of investigation, found out that the LMU file referred to as "Map0077" contained the piece of code which allowed Karolina, in theory, to obtain the Heart Key. (Judging by the text written for "Map0077" it is absolutely conclusive that it is the room with Adrien before you go on through the double doors to meet with Elaine)
However, despite this, much like every single other key, the Heart Key isn't found in a wardrobe, drawer, or somesuch, but as a physical object in the game. However. I have checked every walkable pixel in that room and I can conclusively say that there isn't any Heart Key there.
This is certainly a bug, since there's no real way to obtain the Heart Key, and, again, after checking the game files, I have found that "Map 0137" is supposed to be the room that opens up to Karolina after she has obtained the Heart Key and used it on the Heart Key doors.
Due to the player not being able to obtain the Heart Key, however, that section of the map is completely locked down to the player.
I would very much appreciate if this slight overlook would be fixed, thank you!