1) The fianchettoed bishop is more to improve its attacking range rather than defensive, if we assume the pawns are endeavoured to be put on the same squares as the fianchettoed bishop, then the pawns usually do a good enough job in conjunction with the knights.
2) Usually you'll see the kingside fianchetto more because it helps to protect the king.
3) If the opposing player fianchetto's their queens bishop, then usually it is a good idea to counter fianchetto on the kingside. Sometimes if the opponent has spent the time to play moves like h3 or h6, then your bishop is sometimes better on b7 or b2.
It's well understood that the purpose of the fianchetto bishop is to put pressure on the center by controlling rather than occupying the central squares (hypermodern opening theory).
However, the kind of control seems to vary by opening considerations - sometimes control seems to mean attacking central squares that the opponent plans to control, and sometimes control seems to mean protecting your own claim to the center.
If we are playing out of book and sheerly according to opening principles, then:
1) In general, is the fianchettoed bishop supposed to serve its center control function by attacking or protecting the center?
2) What principle(s) govern which bishop should be fianchettoed, or preferred?
3) What considerations, if any, would prompt interest for fianchettoing both bishops?
Thanks!