An old trick is to put the tiniest spot of liquid washing up detergent on the tip of the fipple. This reduces the surface tension, and makes the water flow easier. Another trick I found with my all metal whistles was to pre-warm them in cold weather.

Over-blowing is what gives you the overtone series, the first of which is the octave. You can only blow a fipple instrument higher in pitch, not volume. An instrument can be played slightly louder or softer - I find changing the shape of the inside of the mouth helps a bit, but it it mainly 'bends the pitch'. if you want ot play a differing range of volume, you need more than none instrument.

As for your tune selection allowing you to sound better than you think you are, this is an old secret for performing musos. Us experienced ones could tell you, but then we'd have to kill you!

:-)

Robin