My read is that the problem is upstream of the engines and their FADECs
is this not so?
My understanding of FADECs in general is that their fail-safe mode "if" they go tits up, is to lock the fuel at the last known values according to aircraft sensors like speed,altitude and temperature.
and as for checking software throughout the fleet, might it be as simple as checking what software versions are displayed ?
I may have some questions but I don't know what the facts really are in this machine.