Hello... basially a Computer part is made of transistors that turn off and on to control current from your battery.
SO... any method to keep a transistor from switching will reduce current flow.
1) Yes
2) Yes, but modern GPU's have already 3 Power/mhz reducing steps programed into them.
3) Use CPU Speed Step instead, like the above mentioned, Power/mhz reducing steps programed into them.
4) Yes, if there is LED/Laser lighting being used on them.
2) While yes not powering the gpu would help life, but then your computer wont work or display anything