Bullet 1: Because FADEC eliminates any mechanical connection between thrust lever and fuel control system. Part of the attraction is elimination of many yards of control cable or teleflex (push-pull cable) runs. The other attraction is the computer does a better job.
Multi-channel computers are running the engine. The pilot input when the thrust levers are moved is just a command or request to the computers. As with flight control fly-by-wire, safety and redundancy is achieved with multiple redundant computer channels, sufficient to bring the safely level to equivalent to having a manual backup system.
Bullet 2: No. The systems are completely separate.
Bullet 3: No. An engine monitoring system just monitors and displays or stores information, with no active control. FADEC is complete control of the engine by computers.
Historically, there are basically 4 main engine control evolutions:
Primitive (1940s) - The first jet engines had not much more than a lever that operated a valve on the fuel controller and the pilot's eye-brain-hand interface managed the engine parameters. If you weren't very careful with inputs, you could overtemp or flame out an engine easily.
Modern Hydromechanical (1950s) - You move the trust lever and it moves a lever on the fuel control unit and and the resulting fuel flow is the result of the input, but with various internal hydraulic/pneumatically sensed adjustments to regulate the engine's response. These systems allow you to shove the thrust lever around aggressively without so much worry about flaming out or melting the turbine.
Electronic Engine Control (1970s) - You move the thrust lever, and it moves a lever on a hydromechanical fuel control unit, which commands a "baseline" fuel schedule. This schedule is added to or supplemented by computerized trimming input to get the final fuel schedule. A computer controlled the last, say 5%, of the engine's fuel flow using a torque motor or some other electric servo device on the fuel control unit. The computer was able to fine tune the fuel flow, providing a bit more efficiency and faster response compared to a manual fuel schedule. If you disable an EEC equipped engine's computer, the power will roll back a bit to the untrimmed manual fuel schedule. So if N1 is 88%, and you switch off the EEC or it fails, the N1 might roll back to 84% and you'd have to move the thrust lever up a bit to regain the the original thrust setting, and the engine would be a bit more sluggish in its response.
Full Authority Digital Engine Control (1990s) - No mechanical connection at all. Same as FBW flight controls. You move the trust lever and it's just sending a voltage to a computer, telling the computer you'd like some juice, and the computer decides what it's going to do with fuel flow to give you the juice you want. For safety you will have 2 or more independent computer channels cross checking each other with one in charge and the other monitoring and ready to take over; whatever kind of back-up architecture is required to meet the minimum safety and reliability level.
