Technically speaking, 0 Faith is similar to "infinite MDef", in that it just reduces all magic damage to 0. In FFT, status also hits this MDef, so they would also be 0%. I mean, if you took someone from a game who just had say a million defense and nothing could do more than 0 damage to him, that's not physical immunity. ITD physicals work just fine. 0 Faith is similar to that.
On the other hand, the Innocent status is more like magic immunity. It just basically say "anything that hits MDef is nulled, regardless of MDef value". In FFT, this is basically every attack flagged as "magic", including recovery spells. So it's basically magic immunity.
So for Firefly, I'd probably just say Worker 8 has 0 Faith as normal, but no Innocent. I'd say that any magic attack that is ITD hits him just fine (and remember, that bypasses his innate Defense Up), as well as status that explicitly ignore MDef (or checks some other stat).
As for healing? I'd say if the healing doesn't hit MDef, W8 can get healed due to no Innocent, but if it hits MDef, he can't (since his MDef is infinite). On the other hand, you can also say that FFT MDef affects healing and W8 can't be healed because of that. It's a weird situation I can see going both ways, honestly.
That's what makes most sense to me, anyways.