The Roman Numeral Analysis for Nachtwanderer (mm. 14-17) by Fanny Mendelssohn is... (don't hate me if I'm wrong!)...
m.15:vi bVI V
m.16: viio7/V V
The embellishing scale degree in measure 15 is merely a chromatic passing tone, but it plays a large role in the harmonic movement and continuity of this section. It serves as a transitional point as well, where the piano foreshadows the upcoming viio7/V chord in m.16. In measure 16 Hensel restate the lyrics “Then everything is gray and quiet” which automatically enhances the depth of emotion felt in this passage. Hensel then takes the sense of melancholy even further by using the aforementioned viio7/V chord, wich is minor and therefore generally gloomy by nature. This chord is not a use of modal mixture, but is just as affective. At first, I thought it was a ivo7 chord, which would make it modal, but decided against it because there are no flats which are a part of the fm key signature (I really hope I’m right on that one!) Another argument is that the chord then resolves to a V chord.