I'm thinking that the force guided Finn to being a good guy
While the force does flow through everyone, I think Finn is probably just as force sensitive as say Han.
Rey is really the most force sensitive, when she touched Luke's lightsaber, all the visions came to her mind, but when Finn took the lightsaber nothing happened.
As for Rey's relationship with Luke, I think it is probable that it is a father/daughter dynamic, but I doubt Mara Jade will be in this saga.
But I think Rey is so quick to handle and control the force not because she has 2 force sensitive parents, because it is said that Luke is the last jedi, but because Luke is the most powerful jedi arguably of the entire saga.