The law of liberty is the royal law, which is 'love your neighbor as yourself'. Having respect of persons violates the royal law. The law of Moses nowhere mentions showing respect of persons. John says if we keep the royal law, we will do well, which means that GOD is pleased. He mentions the 10 commandments, under which one has to be perfect (bondage which Peter said they nor their fathers were able to bear) merely to draw a contrast with the law of liberty under which we don't have to be perfect.
Lev 19:33 And if a stranger sojourn with thee in your land, ye shall not vex him.
Lev 19:34 But the stranger that dwelleth with you shall be unto you as one born among you, and thou shalt love him as thyself; for ye were strangers in the land of Egypt: I am the LORD your God.
Hmmm, seems to me there was one Law for Israel and the stranger among them.
As far as being perfect...
Mat 5:48 Be ye therefore perfect, even as your Father which is in heaven is perfect.
One must understand what perfect is.
Now maybe you will show me where it says they had to be perfect, us not so much.