I would say yes the Emancipation Proclamation was justifiable, because before the slaves were prevented from making contracts, gaining rights, etc., which they were mentally capable of. Disallowing them rights was an initiation of force, in the same way as if a man with a shotgun prevented people from going into a polling booth.
I would completely disagree with the statement that all laws are ethical by definition. Unless, of course, we have radically different ideas about ethics, which is plausible.
Well sure, rights and ethics are both human constructs. But what does that change? Why is that so significant?