chopin's nocturne

Chopin’s Nocturne no.20 in C-sharp minor, Opus Posthumous was a nocturne written by Frederic Chopin. This piece first major phrase revolves around a simple progression of V6/5, i, ii4/3, V4-3, and V in the key of c# minor. The second theme has modulations and interesting approaches to come back to its home key. The introduction gives hints of what to expect on the piece and the closing theme could be debatable.
The first 4 measures are the introduction of the piece where it starts with the tonic, then the predominant and introduces a Ger6/5 that emphasizes the dominant with a descending bass. Two major Themes are present throughout the nocturne (m. 5-20 and m. 21-46). The first theme, introduced in m.5, starts with the first and fifth of the tonic chord. The first chord change in this theme occurs in the third beat of the measure where it moves from a i5/3 to a ii4/2. The change from the tonic to the supertonic is more of a passing chord as it goes back to the tonic. In m.8, the melodic line jumps to the first scale-degree where it stays until the third beat where it goes down again, this time to the sixth scale-degree. The harmonic progression of this measure could have two options. It could either be analyze as a major I chord or a V/iv. If we look ahead of the piece to m. 49 to 50, we can notice a variation of this theme. It is clear in the melody and harmony that measure 50 is a iv chord, therefore it would make sense if m.49 was a V/iv chord resolving to the iv in m.50. The D# in m.8 could be considered as a non-related note and therefore the harmonic progression of m.7-8 would be V/iv, V7/iv, and iv. But, it can also be analyzed as a progression from a major I, major I7, and ii7 with a missing third. If we change m.8 to be a ii7 chord, the previous measure with V/iv, v7/iv would be presented as an unresolved secondary dominant.
The antecedent begins in m.9 with a V 6/5 chord, moving into the tonic in the following measure and then changes to ii4/3. In