bombarrific, very interesting ideas of Radagast.
Now, I don't see any reason to doubt Tolkien when he says that Gandalf was the only wizard to stay faithful to his task. I think Radagast strayed from his purpose as the quote you provide shows. The reason the Istari were sent to Middle-earth were to unite the free peoples to fight and defeat Sauron. Of the Wizards, the only one for sure, and the only one we are told to do this is Gandalf.
Gandalf runs around Middle-earth dealing with every race, dwarves, a friend of elves, men, hobbits, Ents, and getting them all to try their own part in defeating Sauron.
While Radagast was a good-hearted wizard, and helped out Gandalf at times, as the quote shows he forsook his task and took to birds and nature.
You bring up Yavanna's trust in Radagast and wanting Radagast to also be one of the wizards. What's interesting is Yavanna is the Valier of nature, animals, and birds...etc This is just speculation also on my part, but perhaps Yavanna wanted Radagast sent as well, because he would take care of the birds and...etc on Middle-earth. And Radagast just fell in love with it so much that he strayed from his true purpose, which was to help the Free peoples of Middle-earth combat Sauron.