Rubato

But those diatonic notes have no real bearing on the tonality as they aren't structural, if you follow the bass it makes a clear step wise decent towards the sub dominant then wraps itself around the dominant before leaping back to the tonic. The diatonic chords are passing between these structural points just as much as the non diatonic ones, calling it a passing chord with no context is just bad terminology.


not necessarily no, it would have moved away from the tonal center but cannot abandon it without establishing a new center or using multiple non functioning chords, you could even write a few measures in a different region without losing the initial tonal center.

SIRIUSB

So, if you were to write a melody in A Major over your progression nothing would sound out of key? I think it would because of the secondary dominants you've placed in between the diatonic progression. Unless of course you adjust the melody to follow the chords, but that would be straying from the tonal center.

Rubato

There would be a high risk of running into cross relations but yes you could. I detest any attempt at separating melody and harmony anyway, both should be considered simultaneously.

Howard the Duck

it could be called a "turnaround" in blues, your ears will still focus on the diatonics

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