Why did he attribute the warning not to harm Ya'akov to "the G-d of your father" and not to the G-d of Ya'akov himself?
Seforno: Because he did not consider Ya'akov sufficiently worthy of Hashem appearing to him on his own merit, since he had stolen his heart and fled without informing him.
Why is Lavan telling Yakov that Hashem told him not to harm him?
Bechor Shor: He shows him that he did foolishly. I have power to harm you. This is why Hashem needed to warn me not to!
Who did Lavan mean that he has power to harm them?
Tosfos ha'Shalem (3, citing R. Efrayim): He could harm Yakov and his slaves. He would not harm his own daughters and grandchildren.