I agree with the foreshadowing. Maybe one of the boys gets a bad day and they think Main Girl isn't there, so he reverts to his original personality and like destroys stuff around him and Main Girl kinda sees and is confused because he is "so nice" to her and then another one of the boys cover up for him with some lie or another like "he was orphaned at a young age and many people were mean to him" or "his dad abused him when he was younger." Just make sure the "lie" which Main Girl see as truth gives the boy a "tragic past" so the readers wouldn't be like "WHY IS HE EVIL?? THIS WAS SO UNEXPECTED" or "I KNEW HE WAS EVIL HAH" but more like "Is he a good character or bad character? Something in my gut tells me tells me something is off but....." Then later, you could make a scene between the boy with the bad day(I'll call him Boy 1) and the boy who came up with a lie for him(I'll call him Boy 2) and have Boy 2 reprimand Boy 1 for almost spilling the beans and stuff. Main Girl could also see this event.
Maybe there should be a drunk or sick scene where one of the boys tell her the truth about how they're going to eat her but in a really mysterious and indirect way so she doesn't take the hint and run to the Alps-equivalent of that world.
Also, I think there's some untapped potential with Main Girl's ex-boyfriend. It would be cool if like only broken-hearted girls can get into the magical alternate world, so the ex-boyfriend was sent to earth to seduce her then break her heart and in exchange, he made a deal to the guys in the other world that he'd get her left leg or something. I dunno, you can trash this idea if you want to, but it's something to think about.
I look forward to reading this novel, but before you publish it, you should close this thread and designate a chapter where you tell your readers not to go to this thread and post a link of it. Sure, some would click the link anyways because YOLO and drop the story once they find out about the ending, but there'll also be the people who wouldn't so they don't get spoiled. Either way, the point of this is so that a reader won't find this and get mad that they accidentally spoiled themselves.
Best of luck 