Hi markmp,

On question #1, I'm not sure I understand. How big of a gap are you seeing? 1/8" 1/4" 3/8" 1/2" greater? Is the gap behind the post or right next to the post? (A picture would tell a thousand words here)

On question #2, if you're screwing into OSB, that's a lot better than just drywall which would be normal on a stickframe interior. I would think that using construction screws would do the job pretty well.

-srj