The King said:I don't see what your quotes bring to the discussion.
Thoth-Amon was an occasional slave because he lost his ring. Moreover tt is not because slavery can be forbidden in Aquilonia, that some won't keep slaves.
Murder is forbidden in the US and most countries inthe world but there still are crimes. Slavery is also forbidden in industrialized countries but there is reported cases in Europe of individual keeping clandestine immigrants as slaves for domestic labour (i.e. people for the Eastern Europe or Africa or Asia).
You should know that it is not because a law is in force that it is rightfully applied everywhere.
You are being anachronistic. In an age where slavery is accepted in most countries -- Zamora, Nemedia, Argos, Zingara... the presence of a slave is evidence of institutionalized slavery if there is no evidence to counter it. You are trying to prove something not supported by Howards text.