The ring as a test of character? It may be that for a while, such as it was with the Hobbits, but ultimately, I believe it is Tom's being or intrinsic nature that is defining.
I agree that it's Tom's nature that is defining in his unaffectedness by the Ring - but rather his human nature so to speak (or rather his character... struggling to find an English equivalent for the word in my native language). All in all, because of the unique characteristics given to him in one of JRRT's letters, as an observer who unlike others doesn't wish to 'do' anything with the knowledge he has. For me that's quite independent on his origins, on 'what' he is, and he can happily be a Maia of a one-of-a-kind temperament for me.
She knew it was more powerful than her will or her nature. As it should be with Maiar against Noldar -
I'm not sure I understand you - what should there be with Maiar against Noldor?