Tried playing through it a number of times, well done so far! Updates can't come fast enough for it. :-)
I tried all sorts of different combinations of options on the playthroughs, and it always comes to the investigation starting with Marc/Lucas. I'm guessing that's scripted as part of the story? I presume who the MC actually kissed, or was kissed by, is influenced directly or indirectly by the player as they play? And if so, are the only options the guys? Rebs early into the investigation straight up says that maybe it was her, but she can't remember after having too much to drink.