I think Finarfin would be a Gryffindor Primary, Slytherin Secondary.
Gryffindor Primaries are committed to what they believe in and won’t tolerate hypocrisy. I see this as the main reason Finarfin turned back during the Flight of the Noldor. They leave to battle Morgoth and gain their freedom. When he turned back, I believe Finarfin felt that to leave Valinor would have been running away from his problems instead of facing them. In his mind, the Kinslaying showed that instead of bringing Morgoth to justice, they were just sinking to his level, his methods. The Doom of Mandos meant that they would just be leaving one cage for another. After that, to continue would have been hypocritical.
He doesn’t give up on his initial goals, he just finds more effective and palatable ways to achieve them. He turns back, but assumes the kingship of the remaining Noldor in Valinor, allowing him to address the wrongs at Alqualonde and ensure the safety and freedom of his people. When the War of Wrath starts, Finarfin leads the Noldor of Valinor into battle.
He turns back even when his brother and children refuse to. Unlike a Slytherin Primary, who would protect their friends and loved ones first, Finarfin does what he believes to be right, even if it means he won’t be able to protect or guide his children. And thanks to the Doom of Mandos, he knows they would need it, too.
Slytherin Secondaries are versatile and I think Finarfin shows a willingness to change plans when he feels necessary. The line that most convinces me is: “Slytherin secondaries go with the flow of the situation and do what they need to in order to take that situation where they think it should go.” While most argue after Feanor’s speech, even coming close to physical violence, Finarfin “spoke softly, as was his wont, and sought to calm the Noldor…”
I think this implies he was often in the position to ameliorate the more passionate members of his family. During the Flight of the Noldor it says that Fingolfin and Finarfin are leaving in part to protect their people from Feanor’s rash decisions. I think Finarfin went also to protect their people from Fingolfin’s own rash behavior.
Finarfin isn’t the immovable object that is the Hufflepuff Secondary, and he’s not the unstoppable force that is the Gryffindor Secondary. He doesn’t view the world as a puzzle like the Ravenclaw Secondary. Instead, I think Finarfin has an acute understanding of what makes people tick, a skill he learned from years of putting his own pride and feelings away to play the peacekeeper for his more impulsive and hot-headed relatives.