BOAC
Yes, countless people do forget the 'errors' of the VOR signal, although I have never seen anything like +/- 5 despite the caveat.
You should have got out more, dear boy - Khartoum, Addis Ababa, and almost anywhere in West Africa all spring to mind.
FD
momentary drift of the commanding IRS
Surely if the AP is engaged the guidance bit of the FMGC is providing steering commands - not the IR? And will it make any difference to the accuracy?
and
OTOH, TRK-FPA (or bird) is designed for NPAs
I tend to think of it as designed for raw data flight, not just NPA's.