doesn't matter if he is a clone or not the fact that he is more powerful with the force then Vader or Skywalker should have been a huge tip off.
Yoda wants to destroy the sith and this guy just strolls on through with the ability to pull a Star Destroyer out of space? Even Yoda can't do that.
Of course then we have the opening crawl of A New Hope which invalidates anything that could be considered a victory for the rebelion.
What did you expect Yoda to do? He can't train him as a Jedi, he's already been trained in the ways of the Force, and he can't tell him to go fight the Emperor and Darth Vader, because he already is doing that. All Yoda could do is let him follow his own path and let the Force guide him, and that's what he did. Your right about the opening crawl though, the Battle of Kamino seems to conflict with it. But hey, there must be countless conflictions in the Star Wars universe and this is a fairly minor one, after all the opening crawl is sort of an out of universe thing. The events said there can be easily altered (there is no actual dialogue in the film that validates what is said there).