Peregrine, you are quite right about the Blue Wizards' mission. They went to the East to help the Men there to get free of Sauron's dominion. We can't be exactly sure whether they failed or not, as Tolkien's texts leave room for both possibilities and they suggest both.
In the Unfinished Tales, we have the story that they probably failed and established some "magic cults" which outlasted Sauron's reign. The Peoples of Middle-Earth (HoME 12), on the other hand, tells us that they put up succesful resistance movements in the East, which drained Sauron's resources so that the western people didn't face Sauron's whole might, and thus allowed them to win the war. This text also gives different names for the Blue Wizards (Morinehtar and Romestamo). Either Tolkien couldn't make up his mind, or he simply forgot what he had written earlier.
The Wizards can be killed (or to be more accurate "permanently disembodied"). When they took up the mission, they had to agree not to use their full Maiarin powers, and they had to take up Mannish bodies and all the troubles the body brings with it (such as hunger). My impression always was that theit powers were limited in other ways as well, not only by an agreement. Saruman, at least, was unable to reform himself.
Gandalf's return was a special case. He didn't pull of the recovery himself, he was aided by Eru. Eru took Gandalf "out of time and thought" and enhanced his powers and then sent him back without the usual wizardly restrictions. That's why Gandalf the White is so much greater than Gandalf the Grey.
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