Not all, but most. Vent to exhaust otherwise you'd have exhaust going into your intake.
Yep, keeps the turbine spinning and reduces shockwaves travelling back to compressor.
The source of the force that opens the wastegate in our cars is compressed air ("boost" air between compressor and engine). The more pressure in this line, the more/quicker the wastegate opens, therefore if you reduce this pressure the wastegate doesn't open as much/as quick/as early. Bleed valve is an easy way to do this.
Never to exahust afaik, but to either of the others depending on the set up. Stock is back to intake.