[Glirdan]

How about this than: What if Gandalf guessed that, from what Radagast reported to him, that Saruman turned traitor.

OR

get rid of my other theory and start over with a different question which is: what if Gandalf sent the letter to Frodo himself? Than, he could have gone to Isengard and still have been held captive and then the rest of what I said in my other post happens except for the fact that Gandalf is not part of the group.