Third, when the bus transfer relay is positioned 'off' why do the transfer buses lose power? I thought that this just meant that power cannot be transfered from one transfer bus to the other should a generator fail?
This statement is most likely true only when you are supplying the aircraft with Ext or APU power (The bus ties need to be closed to get power from Ext or APU to the Xfr Buses).
Regarding the scavenge question...
The NG scavenge pumps are "hydromechanical" and are not electrically operated and do not have timers.
On the NG, there is a float valve in the Left tank which won't allow fuel to be scavenged from the centre tank unless the fuel level in the Left tank is below 4,487lbs (1990Kg).
Scavenge flow rate is between 220 to 450lbs/hr (100~200Kg/hr)
In your scenario, the scavenging won't begin until the float valve is opened. The scavenge pump should continue to operate until the left tank fwd boost pump runs dry (Obviously the scavenge rate will be insufficient to empty the Centre Tank first).
I stand to be corrected
Rgds.
NSEU