Magic vs. Power

[Salocin]

1) one or more of the Nine became sorcerors at some point after having been given a Ring of Power by Sauron but before becoming a Ringwraith; and

2) Gandalf learned "bewitchments" involving fire and lights which he used to bring lightning bolts down on the goblins in the Misty Mountains and set light to fir-cones to throw at the Wargs, as well as making fireworks.

I think you forgot one piece of evidence we have:

3)Beorn is described as a sorceror and had (at least) skin changing abilities. That ability, though, was most likely inherent in all Beornlings and therefore probably not part of his sorcery. The fact remains that he was a sorceror and, as far as we know, did not posses some item of great power that allowed him to learn as we suspect the Witch King might have.

And, having become great rulers, they would have had the wherewithal to seek out those who were able to teach them magic

Thats all well and good except there is no evidence of people for them to seek out aside from Sauron that is but I don't think he is what you are refering to since they would not have to seek him out.

Oh, here is another bit of evidence we have:

4)The Mouth of Sauron was a great Black Numenorean sorcerer. Aperently thousands of years old (he extended his life by some unknown magic, maybe a connection to the life extending properties that Sauron gave the rings of power?) it is unclear whether he became a sorcerer under Sauron or went into Sauron's service because he was a sorcerer.

There, I think that is all the references to sorcerers in ME.