I think once an umpire calls an infield fly rule all further play is suspended and all runners have to return to their original bases.
That is why the rule was implemented.
Nope. Ball is live and runners advance at own peril.
Nope. After an infield fly rule has been called, runners do NOT have to tag their bases. This scenario seems to rely on one of the runners not realizing that.
The infield fly rule is not in effect if there is a runner on first only. The runner should head back to FIRST base. If the defense drops the ball to get a force out at 2nd, the notion is that the batter would likely reach 1st base safely, so the rule-makers felt there is no motive to purposely bungle the play. They obviously never considered that there would ever be a Rickey Henderson.