[Legolas]

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.

Indeed, of all the Istari, one only remained faithful, and he was the last-comer [Gandalf]. For Radagast, the fourth, became enamoured of the many beasts and birds that dwelt in Middle-earth, and forsook Elves and Men, and spent his days among the wild creatures. Thus he got his name (which is in the tongue of Númenor of old, and signifies, it is said, "tender of beasts"). And Curunír 'Lân, Saruman the White, fell from his high errand, and becoming proud and impatient and enamoured of power sought to have his own will by force, and to oust Sauron; but he was ensnared by that dark spirit, mightier than he. (Unfinished Tales)

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:

[the wizards] were forbidden to reveal themselves in forms of majesty, or to seek to rule the wills of Men or Elves by open display of power, but coming in shapes weak and humble were bidden to advise and persuade Men and Elves to good, and to seek to unite in love and understanding all those whom Sauron, should he come again, would endeavour to dominate and corrupt." (Unfinished Tales)

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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:

It might be (though this is the merest guess) that Oromë of all the Valar had the greatest knowledge of the further parts of Middle-earth, and that the Blue Wizards were destined to journey in those regions and to remain there. (Unfinished Tales)

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.

I have not named the colours, because I do not know them. I doubt if they had distinctive colours. Distinction was only required in the case of the three who remained in the relatively small area of the North-west. (On the names see Q[uestion]5.) I really do not know anything clearly about the other two – since they do not concern the history of the N.W. I think they went as emissaries to distant regions, East and South, far out of Númenórean range: missionaries to 'enemy-occupied' lands, as it were. What success they had I do not know; but I fear that they failed, as Saruman did, though doubtless in different ways; and I suspect they were founders or beginners of secret cults and 'magic' traditions that outlasted the fall of Sauron. (Letter 211)

2) as the Blue Wizards or Ithryn Luin in the 1954 Istari essay in Unfinished Tales:

...the chiefs were five. [...] Others there were also: two clad in sea-blue, [...]Of the Blue little was known in the West, and they had no names save Ithryn Luin 'the Blue Wizards'; for they passed into the East with Curunir, but they never returned, and whether they remained in the East, pursuing there the purposes for which they were sent; or perished; or as some hold were ensnared by Sauron and became his servants, is not now known. But none of these chances were impossible to be; for, strange indeed though this may seem, the Istari, being clad in bodies of Middle-Earth, might even as Men and Elves fall away from their purposes, and do evil, forgetting the good in the search for power to effect it. (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:

...a council of the Valar, summoned it seems by Manwë [...] to send out three emissaries to Middle-earth. [...] But only two came forward: Curumo, who was chosen by Aulë, and Alatar, who was sent by Oromë.

The note ends with a statement that Curumo took Aiwendil because Yavanna begged him, and that Alatar took Pallando as a friend. [...] There are here also some rough tables relating the names of the Istari to the names of the Valar: [...] Alatar to Oromë, and Pallando also to Oromë (but this replaces Pallando to Mandos and Nienna). (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.

The 'other two' came much earlier, at the same time probably as Glorfindel, when matters became very dangerous in the Second Age. Glorfindel was sent to aid Elrond and was (though not yet said) pre-eminent in the war of Eriador. But the other two Istari were sent for a different purpose. Morinehtar and Romestamo. Darkness-slayer and East-helper. Their task was to circumvent Sauron: to bring help to the few tribes of Men that had rebelled from Melkor-worship, to stir up rebellion ... and after his first fall to search out his hiding (in which they failed) and to cause [?dissension and disarray] among the dark East ... They must have had a very great influence on the history of the Second Age and Third Age in weakening and disarraying the forces of the east ... who would both in the Second Age and Third Age otherwise have ... outnumbered the West. (HoME XII: Peoples of Middle-earth)

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.