@Draconis, the bass notes held by the sostenuto pedal is how my baby grand works as well. I have never seen it do differently, on any piano I have checked. (Keeping in mind that I wasn't fiddling around looking at the pedals and their effects on the dampers when I had opportunity to rehearse or perform on a real concert grand.)

@dfhwze, well I guess I have not played enough Debussy since they haven't been in the Debussy I have played. It could be the publisher changed the triple flats to more standard notation to make it easier to read.

@Caters, yes it would characterize an Alberti bass line as an arpeggio. And why not? What makes it a broken chord and not an arpeggio? All squares are rectangles, but not all rectangles are squares. All arpeggios are broken chords, but not all broken chords are arpeggios.

@MaikaSakuranomiya, yes that is true. I was just describing what I was taught regarding the name.

Good suggestion, but 15 roots would be better to account for the enharmonic keys.

@Caters, exactly. However, this is really only in response to your question about the structure of a musical period. You are not required to stick to that structure. Phrase groups can have all kinds of cadence patterns that don't necessarily conform to the structure of a "period." Heck, I have ended several pieces with an IAC because the soprano/melody ends on the third instead of the tonic. It still sounds final, just in a wistful way. What's most important is the effect that you want, and it sounds like the IAC is doing that for you in this spot.

@MichaelCurtis, I agree that the lowered notes would not be found over a dominant chord. But whether the harmony or melody came first, only the composer can answer that.

@MichaelCurtis, I don't see the "traditional treatment is clearly based in harmony" as clear at all. Melodic minor is used melodically. Bach is full of music where the melody heading down, away from the tonic, uses the natural 6, but on the way up to towards the melody is raised. The chord would be adjusted accordingly to accompany the melody. The IV does not move to V and better than iv to V. I don't see an important functional difference between the two harmonies.

@Tim, either way those chords formed in the melodic or harmonic minor are still essentially borrowed from the major and do not change their function in the chord progression.

@AlbrechtHügli, what else would you suggest? My understanding is that major and minor keys that share the same tonic note are "parallel." But perhaps that is because I am American. I do not know of another term.