A: Here are the most common reasons for a runaway ascent and how to deal with them.
Failing to vent enough air from your BC before you leave the bottom. Always make sure you're slightly negative before you start swimming up.
Not venting air periodically as you ascend. Maintain neutral or slightly negative buoyancy all the way to the surface by venting expanding air from your BC as you ascend--usually every 10 feet or so.
An inflator malfunction from improperly serviced or maintained equipment. If you experience an unusual increase in buoyancy, pull your quick-dump valve/valves (usually located on either shoulder and/or on the lower back of the BC), and hold them open until you find out what's wrong. If your BC's inflator hose is stuck and constantly inflating (an uncommon, but entirely possible, situation), disconnect the low-pressure hose from the inflator as quickly as possible before making a controlled ascent--orally inflate your BC on the surface.
A: Welcome to the club of wishful thinkers. Unfortunately, today's GPS receivers can't pick up satellite transmissions underwater. So, for this to work, you would have to use a surface buoy or boat-mounted receiver that intercepts the data and communicates it to the underwater receiver. The underwater receiver would then need to calculate its position from the surface receiver. For accuracy, this would require triangulation using more than one surface unit, just like a land-based GPS receiver uses more than one satellite. Alternatively, you could hard-wire your underwater receiver to a surface antenna on a towable float. Both of these options are prohibitively complicated for most divers and equipment manufacturers.
There are self-contained (not GPS) navigation devices out there. Dive Tracker, made by a company called Desert Star, is one example. You hang a sonar transmitter in the water near the boat or exit point and carry a hand-held receiver that measures the strength of the signal. The receiver displays rough directions on a graphic display based on how close you are to the transmitter. This type of system won't tell you how far away the boat is, but by following the strength of the signal, you eventually reach the boat or exit point.
A: You'll get the same response. But don't worry, you can cough, sneeze or even vomit through your regulator's second stage without major issues. So, if this happens at depth, just clamp down on your second stage mouthpiece and cough until you clear your airway. However, don't try to go up because coughing can briefly and involuntarily close your airway, putting you at risk for a lung overpressurization injury. Just stay at your current depth until the problem passes.