Boundary Line

[ChuckElias]

1. No
2. No
3. Depends on his location when he touches (or is touched by) the ball. If he has re-established inbound status (something touching inbounds and nothing touching OOB), then it's legal to touch the ball.
4. Kind of a trick question. If the defensive player steps on the OOB boundary, then he has broken the plane and committed a violation. So, although it's not possible to step "on (but not over) the boundary line", I think the answer to your question is yes. A defensive player who steps on the boundary line has committed a violation.