Don't critical SW components need to be n-versioned with voting? I know from many years back (I think) the 744 FADEC was designed this way.
They are normally dual channel i.e A/B with additional cross checking between each seperate FADEC IIRC
I don't think that there would be a software download just before a flight. There's a lot of work before release in testing the software to ensure that it doesn't do anything bad
I would hope that all software testing is done before its released from vendor
Once you fit the FADEC it should not need much more than an Installation run