a corner kick
No, unless the defensive team touches or recovers the ball after the initial block.
The ball is placed on the 2 yard line when kicking a PAT
The 30-yard line of the team kicking the ball off.
Being ahead of the defensive line and the ball, though if a player is ahead of the defensive line but behind the ball, they are not offside
The ball is placed at the original line of scrimmage unless the defensive team touches or takes possession of the ball after the initial block.
Defensive offsides is when a defensive player crosses the line of scrimmage, the yard line on which the ball is placed before the play begins, when the quarterback snaps the ball. The defensive player is allowed to accidentally jump across the line of scrimmage as long as he gets back to the defensive side of the ball before the ball is snapped. Offsides is a five yard penalty and a replay of down.
If the ball goes into the endzone due to an offensive player or action (taking the snap into the endzone, running it back into the endzone, etc...), then the play results in a safety. If the ball goes into the endzone due to a defensive player, such as a kick (once the ball is kicked, the kicking team is considered 'giving up' the ball, and is therefore a defense), then the play results in a touchback to the 20 yard line (25 on kickoffs in the NCAA).
they line up behind the defensive line thats why they are called line backers
Yes, but only if the ball doesn't cross the line of scrimmage. Once it does, the kicking team can still recover the ball but can't advance it.
I assume you mean "field goal." Yes, this is possible, if the ball is blocked and never crosses the line of scrimmage, and the kicking team recovers. The kicking team could also recover the ball past the line of scrimmage if the kicking team touched the ball first.
If a field goal is attempted on any down other than fourth, the rules state that once the ball crosses the line of scrimmage the kicking team no longer has possession. This means that if a field goal is attempted on second down and the kick is blocked and the kicking team recovers behind the line of scrimmage, they retain possession. If the ball crosses the line of scrimmage, regardless of whether the kicking team recovers or not, possession goes to the defensive team.