More to the point, if all the power goes away, what happens to all
the "settings" the FADEC controls?

Do they go to zero, full, stay where they are?

Without electrical power, apart from the fact that the FADEC is a computer,
and no computer will run without power, on a common rail diesel the
injectors cannot open. A purely mechanical "limp home" mode is physically
not possible. Multipoint or direct injected gasoline engines are the same.

It appears that they go to zero, which is a damn unhandy failure
mode.

Yes it is, but that's just the way it is. It's no different from the failure
mode you get when your fuel supply or both magnetos fail or something
similar. That's why the power supply of a FADEC should be held to similar
stringent redundancy requirements. Actually, the FADEC itself IS double
redundant but fed from just one battery. Go figure.

regards,
Friedrich