MC is not "Trying" to do anything. He has no thoughts of having a throuple, nowhere the game gave any idea MC wants to have both of them, he is indeed confused but the thought he could have both of them is not there in his mind, unless i missed something.
MC is not manipulating lorelei, if you mean the scene where glade starts kissing him in front of lorelei in their bed, thats not MC manipulating lorelei or being dismissive of her, he is confused ofcourse and glade is wayy to possessive and marking her territory in front of her competition. And lorelei specifically is not pathetic for it, her nature and her character is miles different from glade. She is not a possessive and outspoken girl, she is shy and reserved, she is very kind by nature, and thats what may give you the idea that she is "pathetic" and getting run over by everyone, but you cannot be more wrong. A reserved person does not equate to a pathetic person.
I'm pretty sure as initially glade is getting the upper hand on the throuple, tides will turn in favor of lorelei soon. Being a pure & kind loving soul does not make a person pathetic. There's a reason why she is the most loved LI in the game.