Hi Boston,
Part of the answer, as they said in Smokey in the Bandit, depends on what part of America you're standing in.
In the NHFL, the interceptor is called out by the defensive coach before the board's turned on, so he's the only one that can make the pick. In other leagues/tourneys around the nation, any unengaged defender can make the pick (if a defender touches it while engaged - it's incomplete, usually).
Unengaged defenders anywhere on the field can be pivoted, but leagues play it differently in terms of pivoting before and/or after the ball attempt.
OLs in the way and such, in all leagues I've played in, CANNOT be turned. You just have to hope he gets his fat butt outta the way.
Hope this helps.
Roo