It is a very interesting analysis. I will throw in some of my observations and assumptions based on various readings. I think this piece in particular should not be looked at, with the usual classical music hierarchical form. There are no real themes and development in the standard sense of Beethoven; instead, Debussy is using motives which instead of being developed are repeated in various forms and with various scales and harmonic context. Debussy was not new in doing this, the Russian school with Glinka, Rimsky-K, Moussorgski already used that approach. Debussy had anyway since a long time decided that form should fit the music and not the other way around and was criticized by many musicians for his formless compositions.
In this piece, Debussy is going one step further and it can be viewed as one of Debussy attempts to create a discontinuous piece of music. Debussy had been influenced by the music of Stravinsky, Petrushka in particular, and also was fascinated by the cinematographic art. In essence in both cases the technic used was to write music as the combination of different strands which appear by sections. In a movie, there are several storylines that are combined sequentially by sections. And even within one storyline, the action does progress by sections (scenes). Some are just descriptive and others contain developments. The different scenes are interconnected, but essentially it is the audience that fills in the blank, so what is in fact discontinuous appears as a unified story with a beginning and an end.
So Ondine is in fact a series of self-contained sections (15, it can be extended to 17 if one looks at bars 30-31 and 42-43 as an independent section – I tend to look at them as a transition) which belong to different lines, each supported by a particular harmonic context based on various scales. Except a few, All the sections are in fact in an harmonic stasis, i.e. they are based on a given scale and pedal point. The exceptions are the sections 20-25 with a very small harmonic activity and then essentially bars 44-53 and 54-62 where all the “action” takes place. Each section presents a short “image”, as a descriptive snapshot of the story. The sections are unrelated to each other from a motivic standpoint but can be linked by their harmonic context within the strand/line. Indeed Debussy, like Stravinsky implemented a number of connectors to “link” the various sections to each other. Some connectors are the repetition of sections, others are harmonic connectors or motivic elements.
Though the piece has some obvious symmetries, I don’t see that as a major factor. Indeed, musically the climax starts around the 2/3 part of the piece. I don’t believe much the calculation around the global ratio (which anyway depends how you count the bars), which was never demonstrated for Debussy (Roy Howat recognizing that there isnt any written formal recognition by Debussy), and essentially, it is not surprising that the climax happens toward the end of the piece just like in a movie.
The introduction is in fact a true introduction. It is not based on a recurring theme, which anyway Debussy was trying to get away from, but it does present all the harmonic elements which the piece will use. There are really five scales supporting the various strands of the story: the D major with various chromatic variants (Lydian, minor, ….), the 2 octatonic scales with collection 1 and 2, the whole tone, and the E flat major with chromatism. In bar 5 appears the octatonic collection 2, and in bar 7 the collection 1. Both are based on an octatonic trichord (0,1,7) in the bass, in particular the (F sharp, G, C sharp) in collection 1 which will be used throughout the piece and as the basis of the section 44-53 in a kind of Lydian F sharp octatonic collection 1. Also, the Dominant 13th can also be viewed as the result of 2 chords mixed together, the actual dominant and the octatonic trichord, another link between 2 sections. The introduction is itself made of an aggegate of independant mini sections, after which we start the story with the first 3 bars section 11-14 in octatonic scale which acts as an interruption and signals the start, followed by the first strand, in D major.
Harmonically, one should not look at D major and E flat major as related harmonies. Like many other composers, Debussy was also trying to go away from modulation in the circle of fifth sense. The E flat can be viewed as a neighbour scale of D major, which gets resolved eventually after the section 62-64 in the octatonic scale featuring a pedal on B flat, the upper neighbour of the dominant A. Indeed for the coda we continue the strand in D major which was interrupted after bar 27 with the same octatonic based section with a pedal on A but already approached by the B flat. as a shortcut, the Coda goes directly to D, again an illustration of the snapshot approach. There is no need to spell out the cadence, the audience will fill in the blanks.
We can see how the different stories each written with a different scale, alternate by sections, each sections featuring a particular motive, and eventually converge toward the coda for a final resolution in D major.
Though the piece features some element of what will be called a “moment form” by some later theorists and composers, it is not completely moment based. There is still some continuity and connections, as well as elements of development, even if not the standard ones.