Two things to keep in mind:
1) An Infield Fly is just like any other fly ball, except the batter is out whether the ball is caught or not.
2) A Force Play is created when a runner loses his right to a base because the batter becomes a runner.
2a) A runner can NEVER be forced out when returning to a base.
2b) Once the batter (or any runner "behind" a particular runner) is out, the Force Play no longer exists.
Putting those together gives the answers to the questions