Could Falcon 9 fairing become glidable / flyable?
Currently the fairing is separated at about 100 km altitude, when the aero-thermal flux drops below 1135 W/m2. So it's not separated outside the atmosphere, the fairing still experiences noticeable drag. 

That means the fairing has to be separated from the stage quickly, to prevent it crashing into the payload. With 2 fairing halves this is easy: the two halves can push against each other (using simple pneumatic cylinders or pyrotechnics), and both will move away from the stage in a straight line. By pushing with slightly more force at the top of the fairing, the halves are pushed in a direction where the wind will make sure halves are pushed farther apart. 

If you were to hinge the fairing, you can't do that any more. You get two events: 


open the fairing. One half hinges open while the other one has to stay in place. 
once it's opened far enough, apply thrust to get the fairing away from the payload. 


You can't do this under aerodynamic load: 


the wind will exert a force on the fairing. One wobble in the wrong direction and the fairing will be pushed closed again. 
the half-open fairing is not stable. The nose cones will be pushed inward by the wind.


So you'd have to separate the fairing at a much higher altitude, which reduces the payload of your rocket by a significant amount. It also means the fairing is separated at a higher speed and becomes more difficult to recover.

Mechanical actions are always a risk. A hinged fairing adds to that risk: the timing is critical, and the actions have to be carried out under load from acceleration.

JWST deployment and Shuttle robotic arms have fewer time constraints, and the actions are carried out without a load.