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).
well that's just it. According to Star Wars canon if anything contradicts the movies it's considered non canon.