Hey Manuel @Jqh73o,
B.4 is a tonic prolongation since b.5 is also in tonic, so the V in b.4 is not functional but extending the established tonic.
I don't know how jazz write it, but if it's not prolongation, let say it's a V chord on a tonic pedal, it will just be notated as V, while the tonic pedal will be considered as a non harmonic tone.
Never heard of tritone substitution or Neapolitan seventh. What's that?
Schubert and Chopin use a lot of N chord in root position, just like Chopin's famous C minor prelude.
It's still an augmented sixth since the "sixth" means the interval, but not the first inversion.
I always find the book "Harmony and Voice Leading" a great book!
Henry