1) The fuel used is measured at the engine by HMU. Thus you can see how much fuel has been consumed and compare that to how much fuel has left the tank. A (large) discrepancy can indicate a leak. This also explains the discrepancy when adding fuel used and fuel in tank together as the fuel between tank and engine is not accounted for and when engine shuts down will return to tank (small increase after shutdown in each tank)
2) There is a solenoid called the FMC "C" Solenoid Discreet which signals to the FMC the position of the BLEED AIR VALVE (not the BLEED AIR switch position on the overhead panel), resultant of the indicated position the FMC indicates N1% for the respective side. There is NO need to have bleed air on for calculations of N1%, as a simple reduction for bleed demand can be made when bleed air is switched off (-0.8 for full thrust, -0.7 for reduced thrust and -0.6 for assumed thrust).
A failure of the Solenoid C can have either of two faults: failed in ON or failed in OFF position (referring to BLEED AIR VALVE ON/OFF).
Imagine a situation here FMC "thinks" packs are ON thus reduces N1% to be set by auto-throttle, which in case of BLEED OFF performance requirement would thus be insufficient. Result is that FMC Solenoid C has failed only PACK ON departures can be done.
More critical is the alternative situation where pack are thought to be OFF thus higher N1 are set through FMC signal to auto-throttle, thus potential for over boosting the engine exists. In this scenario it is thus forbidden to use the auto-throttle during FULL power on engines, such as Maximum Continuous Thrust, Take Off, Go Arounds and Climb, thus these operational limits apply.
Identifying the correct failed signal is vital and can easily be done to look in MAINT pages for APU > INPUT MONITORING and look at PACK SWITCH position sensed by FMC and comparing it when cycling through the positions on overhead panel, they should agree when making changes. If solenoid C discreet signal to FMC is correct but FMC indicates pack ON or OFF N1 continuously when switching ON/OFF PACK switches with a pressurised system, it is most likely a sticky Bleed Air Valve requiring some mechanical attention (lubricating)