Hi MCDHU
I am quite sure that on our fleet AoA will continue to be used for computation of FPAair even when all ADR's are OFF.
As for the poster quoting FCOM, the FCOM is greatly simplified.
The vertical part of the FPV (fpa) is computed using both air and inertial data, same as for the normal baro-inertial vertical speed (Vzbi). Simplified: Baro for long term changes, inertial for short term and damping.
F.P.A. = arc tan(Vzbi/GS).
Hope it helps