1. Switch the standby power switch to off, all the essential radios and instruments should fail, swich to standby they'll reactivate. Remember to return switch to auto position!
2. It's been al while but i'm nearly sure G/A mode is a flight director mode only. If the switch are pressed during an approach it disconnects the A/P drives the A/T to take off power (if armed) and puts a fly-up and straight ahead bias onto the flight director needles.
as for 3. and 4. you might need to get some pilot input on those!
You might find
www.b737.org.uk a useful source of information.