Quote:
Did the Valar really send the Istari?? Or was that just Eru's plan from the beginning.
Eru did manipulate things from time to time during the history of Arda, but when you look closely at the occasions he did so, they generally revolved around Elves and Men. The reason for this is because this "interference" was done through themes in the Great Music, most noticably the third theme (when he introduced the theme of his Children).
As the Istari are Maiar, they actually took part in the Great Music, and are therefore not themes added by Eru to counteract Morgoth's destructive themes. Each time Eru altered or wove new themes, others joined in with him, and therefore when the music came to be played out for real, they acted their part.
I'm finding it a little difficult to explain this concept, so I hope you are able to follow what I am trying to say. Put simply, the Istari are Maian spirits who took up part of Eru's theme and later played the part for real, whereas the likes of Earendil, Tuor, Beren and Aragorn are likely to be elements Eru introduced himself with specific roles to play. They were tools created by Eru, with the sole intention of countering the plans of Melkor and Sauron.