That the trolls didn't reveal the past to Anna when she visited them always strikes me as odd. They're definitely playing for their own interests.
That was what sent Anna off to find Hans, but also the key for Elsa.
Love is at the heart of her control of her powers, not fear. The trolls knew, but instead of teaching her this and helping her learn how to control her powers, they taught her to fear them.
And Kristoff is just a kid, it is possible that his father is one of the iceman(the opening scene cutting ice).