If you're going to go through the trouble of explaining "The Plan" and call the movie The Plan ... then "The Plan" should be at least something more compelling then: KILL ALL HUMANS, INCLUDING INNOCENT HOMELESS BOYS ... <EVIL GLARE> GRRR!!!
You do realize that there's no evidence that the boy actually existed, right? That he could just have easily been a head manifestation. Note the total lack of blood when Cavil murdered him. Or the fact that no one else was shown to have seen or noticed him. Considering that killing the boy was the last step along Cavil's road to damnation -- the moment he let him stay gave rise to the question of redemption for him -- it's very possible that he could have been one of the angelic beings testing him.
What we really saw was that the Cylons were doomed to failure from the very beginning. That Cavil was manipulating everyone, Cylon and Human alike. That he wasn't even above destroying his own model to see his madness through despite he, himself, discovering just how much of a failure his plan was through his own model.
The entire point of the movie was to show that Cavil was a madman. Even his own model saw the light, but he quelled that with an iron fist.
Most of all it showed us that the real plan was God's plan. That no matter what Cavil tried to do in his insanity, the human condition survived. That love was the answer for everything. Love allowed the Cylons and Humans to mate. Love turned Cylons away from Cavil's plan. Love allowed the survivors to make it to Earth despite all the pitfalls that came their way.
Battlestar Galactica was a love story. And The Plan
(note that the title wasn't The Cylon's Plan
or Cavil's Plan
) showed us just how powerful it was.
Regardless of all of that, the movie left us with just as many questions as it answered. The mystery is still there in a number of places. How did Starbuck survive her death and return with a sparkling new Viper that triggered the Final Five? Who was her father? What was going on with Three? etc.