My biggest problem so far is that there are clear "good guys" and "bad guys" among the love interests. Mark is controlling and possessive, and Ken is intensely violent with barely any provocation. Meanwhile, Gabe's just a sweetheart with low self-esteem and Hiro's a loveable nerd. Unless Gabe and Hiro are secretly monsters, I don't feel like I've been given reason to care about Mark or Ken over the good boys. They're still compelling characters, sure, but not very likeable. I'm genuinely confused as to if we're even supposed to be rooting for Grey to get with them at this point. It's entirely possible that we aren't; this is primarily a horror story, not a rom-com. Still.
Ken seems to have a bipolar disorder or double personality. For now i would say he is more of a loose cannon. It's hard to say how much of what he does is him.
Mark though, if he is indeed the perpretator of things the story has implied there's no excuse. Emotional trauma doesn't absolve you from culpability, it merely gives context.
We often equate character routes to romantic routes, but that doesn't have to always be the case. It's not impossible for Grey to not end up with any of the main characters, because that might not be the point to the story. Considering what's been presented so far, the goal seems for Grey to save those characters, potentially from themselves, and in turn learn to value his own.