Those little words (prepositions) reveal direction, the time element, and the location of objects in a sentence. We’ve seen how unto indicates more of a closer, delivered, and sometimes personal relationship, when compared with the ubiquitous to.
Jonah
In the case of Jonah, God first commissioned him to simply go to Nineveh and cry against it. (Jonah 1:2). Maybe Jonah could stand outside one of the gates, or against the outside wall, and from there he could cry out. Surely God knew that Jonah was loath to enter into that great city— he disliked those people; so, just getting there, in the proximity, was a lot! Therefore the preposition to was appropriately used.
But after he had his rebellion, followed by his marine correction, the prepositions of the second commission changed from to and against to unto and unto. And so, where did these new prepositions locate Jonah? Instead of crying against the city, he actually entered into it, a full day’s journey. (I suppose the inside of Nineveh’s walls were a much better prospect than being inside the walls of a great fish). Then he cried out unto the residents, personally (Jonah 3:4). Mission accomplished, message directly delivered.
Esther
“...was written according to all that Haman had commanded unto the king’s lieutenants, and to the governors that were in every province, and to the rulers of every people of every province...” (Esther 3:12).
Why would unto be used for the king’s lieutenants, while to is used for governors and rulers?
The answer is the KJV is accurately locating these objects: lieutenants pertain to the king. This wicked document was personally delivered to (unto) them; they were in the king’s presence. However, the governors and rulers reside out in far-flung provinces; therefore the decree from the king arrived via post or some second-hand method.
Moses/ Aaron
“And the LORD spake unto Moses and unto Aaron...” (Lev. 14:33)
“And the LORD spake unto Moses and to Aaron...” (Lev. 15:1)
If I had to guess why there is a difference, why in one sentence the LORD spoke unto Aaron, and then in another He spoke to him, I’d say the KJV is accurately indicating the object locations with the prepositions. God spake to both men personally at first (14:33), and therefore unto is used for both objects (Moses and Aaron). Later (15:1), I’d guess God spake directly unto Moses, who in turn passed it on to Aaron. Thus, a to locates Aaron, relative to the LORD. And the narrative confirms this—Lev. 16:2.
There are no mistakes; there are no archaic words employed here! The KJV is really a wonderful and accurate work.