Even though it was impossible for Christ to sin, since he is God, was it possible for him to sin in his human nature?
No. Since Our Lord is a divine person, God the Son, he cannot sin. This is the mystery of the Incarnation. There is one Person, God the Son, with two natures, human and divine. The inablility to sin does not take anything away from the humanity of God the Son, since sin is not a necessary element of any being. On the contrary, sin is an anomaly, a mistake, a deformation of what should exist, not a natural necessity. So Our Lord could not sin in his human nature, but this quality makes him more human not less human.