This is a pretty popular topic; searching the site should give you several threads on the other wizards. I also wrote a
FAQ on it sometime ago. The topic of these other wizards has always been interesting to me, but I've been disappointed at the outcome and lack of detail given. They were clearly low priority for Tolkien as they had only limited (Radagast) or no (Alatar/Pallando) involvement in his main narrative. Given enough time, he may have eventually decided what happened with the blue guys.
As for the other Istari not fulfilling their purpose, Tolkien says as much in the quote above (direct explanation for Radagast, indirect for the Blue Wizards). The Istari quest was a singular mission from Manwë - to guide the peoples of Middle-earth to defeat Sauron. Any other objectives were not 'secondary' missions present on Manwë's agenda (arrived at by counsel with Eru), but misguided and unapproved personal goals of the other Valar or the individual wizards.
Their mission:
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Failure - particularly in Radagast's concern - does not necessarily mean turning to evil. As you say, it seems Radagast may have been sent by Yavanna with an ulterior motive, but he certainly failed in regards to the Istari mission which was to guide Middle-earth's inhabitants to deal with Sauron. I do not think Yavanna sent Radagast with Saruman for any other reasons; if he was sent to supervise Saruman, they obviously picked the wrong sort of fellow as from the start, Saruman resents his presence and seems to regard him as lower in intelligence or power. ("Radagast is presented as a person of much less power and wisdom" [than are Saruman and Gandalf].)
Unwittingly Radagast still played a positive part in the story. Upon warning Gandalf of the threat of Nazgul, he sent Gwaihir who ended up saving Gandalf from Orthanc (and of course, showing up again later). Perhaps Yavanna suspected that the animals of Middle-earth (particularly eagles) would continue to have important moments as they did earlier in history.
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Like the Radagast/Yavanna/nature connection, Christopher Tolkien observes a similar connection (if not completely ulterior motives) in Alatar and Pallando being Maiar of Oromë:
Early in Arda's history, Oromë rode all about the world on his horse, including the east where he found the elves while still sleeping. It could be no coincidence that 'his' two wizards wander off in the same direction.
While you suggest that they established certain spiritual or magical practices, those are not heralded as positive things as Tolkien speculates on their actions. It's important to remember that 'Alatar' and 'Pallando' are never even mentioned by name in anything Tolkien published nor are they in
The Silmarillion, so these names are not fully canon. They are only known to have existed by Saruman, Gandalf, Radagast, Cirdan, and probably Elrond, Galadriel, and Celeborn. As readers, we were only alerted to the existence of the fourth and fifth wizards by Saruman's reference to "the rods of the Five Wizards" in his confrontation with Gandalf. I suspect the names 'Alatar and Pallando' are the most widely known because they were first to be revealed (1980,
Unfinished Tales), and stuck because they are more pronounceable than Morinehtar and Romestamo (1996,
History of Middle-earth XII: The Peoples of Middle-earth).
These other two wizards are only described in any fashion on four occasions - a letter, two notes in
Unfinished Tales, and one passage in
HoME XII.
1) without name or color in a 1958 letter (#211) response to a curious reader. The reader asked what color the other two wizards were, and Tolkien replied he did not know - he actually did have ideas but because he was undecided and nothing had been published on them yet, he didn't commit to anything.
2) as the Blue Wizards or
Ithryn Luin in the 1954 Istari essay in
Unfinished Tales:
3) as Alatar and Pallando in a slightly later 'hasty sketch' of the council held to choose and send the Istari, summed up by Christopher Tolkien in
Unfinished Tales:
4) as Morinehtar and Romestamo in
History of Middle-earth XII: Peoples of Middle-earth, which I believe was abandoned, but I cannot find my copy to look at a possible date on this excerpt. It clearly does not agree that much with the other three statements, having them show up during the Second Age and with these names unique to this passage.
Quite the wall of text, I know. I weighed over this a while, but maybe the information will be useful to some. I'll catch up to the conversation in the next post.