Yes the kid was Hayekawa, we saw the little boy transition into Hayekawa in one of the scenes.
I think Makima is definitely just playing everyone. She is very good at manipulating others to achieve her goals. So when Denji is demotivated because he doesn´t know if it´s worth fighting for a "dream" because getting a feel wasn´t as good as the anticipation she goes all out in giving him new motivation and a new goal to work towards. She knows what she is doing.
And I don´t think she has any genuine interest in anyone, they are just useful to her