How do you know it's for focus? The screws that one can normally adjust are for laser power. Adjusting power can partially hide some focus problems, but it won't fundamentally solve the problem of a misaligned track/bent frame.
If the screw was on the sliding optical assembly itself, I doubt very much it was for focus. Again, if it were a focus problem, it would cause trouble everywhere, not selectively. It sounded from your description that you were getting reliable reads on the inner tracks, so that makes me doubt that you had a focus problem.
As for DVD vs CD, older drives had just one laser for both (a red one). That created problems with reading writeable media (except CDRW, interestingly enough). Newer drives have two lasers -- one red, one IR -- to handle all media types without trouble.
If the screw was on the sliding optical assembly itself, I doubt very much it was for focus. Again, if it were a focus problem, it would cause trouble everywhere, not selectively. It sounded from your description that you were getting reliable reads on the inner tracks, so that makes me doubt that you had a focus problem.
As for DVD vs CD, older drives had just one laser for both (a red one). That created problems with reading writeable media (except CDRW, interestingly enough). Newer drives have two lasers -- one red, one IR -- to handle all media types without trouble.