A: The bottom of the Treasure Map card says “This card cannot be Plundered, but if it is lost or stolen, the Booty goes with it.” This indicates specifically that you CANNOT save your hidden Booty if the Map is stolen. Being able to “reveal [Booty] at any time” does not include retroactively deciding to have already dug up your treasure when you realize that someone is about to take possession of the map.
Once someone plays Steal A Keeper, the transfer of ownership is considered to take place instantaneously; there’s no time in between playing that card and them owning your Map during which you could reveal those cards. The only thing you might do after Steal A Keeper is played is play the Surprise that prevents Actions. Surprises are the only cards intended to interrupt or retroactively prevent what is normally an instantaneous effect.